Add a check box for lossless compression

This makes it simpler for users to discover how to use lossless
compression.
When lossless compression is enabled, the quality slider controls how
much effort libwebp will put into compressing the image data; smaller
values are faster but produce larger files.

This replaces the previous behavior where lossless compression would be
silently used when the quality slider was set to 100.

Fixes #12

src/Properties/Resources.resx

@@ -193,4 +193,7 @@
   <data name="UnsupportedWebPFeature" xml:space="preserve">
     <value>The image uses a WebP feature that is not supported.</value>
   </data>
+  <data name="Lossless_Description" xml:space="preserve">
+    <value>Lossless</value>
+  </data>
 </root>

src/WebP/WebP.cpp

@@ -199,7 +199,7 @@ int __stdcall WebPSave(
     config.method = 6; // 6 is the highest quality encoding
     config.thread_level = 1;
 
-    if (encodeOptions->quality == 100)
+    if (encodeOptions->lossless)
     {
         config.lossless = 1;
         pic->use_argb = 1;

src/WebP/WebP.h

@@ -41,6 +41,7 @@ typedef struct EncodeParams
 {
     float quality;
     int preset;
+    bool lossless;
 }EncParams;
 
 enum MetadataType

src/WebPFile.cs

@@ -123,6 +123,9 @@ internal static unsafe Surface Load(byte[] webpBytes)
         /// <param name="output">The output Stream.</param>
         /// <param name="quality">The WebP save quality.</param>
         /// <param name="preset">The WebP encoding preset.</param>
+        /// <param name="lossless">
+        /// <see langword="true"/> if lossless encoding should be used; otherwise, <see langword="false"/>.
+        /// </param>
         /// <param name="scratchSurface">The scratch surface.</param>
         /// <param name="progressCallback">The progress callback.</param>
         /// <exception cref="FormatException">The image exceeds 16383 pixels in width and/or height.</exception>
@@ -133,6 +136,7 @@ internal static void Save(
             Stream output,
             int quality,
             WebPPreset preset,
+            bool lossless,
             Surface scratchSurface,
             ProgressEventHandler progressCallback)
         {
@@ -144,7 +148,8 @@ internal static void Save(
             WebPNative.EncodeParams encParams = new()
             {
                 quality = quality,
-                preset = preset
+                preset = preset,
+                lossless = lossless
             };
 
             scratchSurface.Clear();

src/WebPFileType.cs

@@ -32,7 +32,8 @@ private enum PropertyNames
             Quality,
             KeepMetadata, // Obsolete, but retained to keep the value from being reused for a different property.
             ForumLink,
-            GitHubLink
+            GitHubLink,
+            Lossless
         }
 
         private readonly IWebPStringResourceManager strings;
@@ -201,6 +202,7 @@ public override PropertyCollection OnCreateSavePropertyCollection()
             {
                 StaticListChoiceProperty.CreateForEnum(PropertyNames.Preset, WebPPreset.Photo, false),
                 new Int32Property(PropertyNames.Quality, 95, 0, 100, false),
+                new BooleanProperty(PropertyNames.Lossless, false),
                 new UriProperty(PropertyNames.ForumLink, new Uri("https://forums.getpaint.net/topic/21773-webp-filetype/")),
                 new UriProperty(PropertyNames.GitHubLink, new Uri("https://github.com/0xC0000054/pdn-webp"))
             };
@@ -224,6 +226,9 @@ public override ControlInfo OnCreateSaveConfigUI(PropertyCollection props)
 
             info.SetPropertyControlValue(PropertyNames.Quality, ControlInfoPropertyNames.DisplayName, GetString("Quality_DisplayName"));
 
+            info.SetPropertyControlValue(PropertyNames.Lossless, ControlInfoPropertyNames.DisplayName, string.Empty);
+            info.SetPropertyControlValue(PropertyNames.Lossless, ControlInfoPropertyNames.Description, GetString("Lossless_Description"));
+
             PropertyControlInfo forumLinkPCI = info.FindControlForPropertyName(PropertyNames.ForumLink);
             forumLinkPCI.ControlProperties[ControlInfoPropertyNames.DisplayName].Value = GetString("ForumLink_DisplayName");
             forumLinkPCI.ControlProperties[ControlInfoPropertyNames.Description].Value = GetString("ForumLink_Description");
@@ -239,8 +244,9 @@ protected override void OnSaveT(Document input, Stream output, PropertyBasedSave
         {
             int quality = token.GetProperty<Int32Property>(PropertyNames.Quality).Value;
             WebPPreset preset = (WebPPreset)token.GetProperty(PropertyNames.Preset).Value;
+            bool lossless = token.GetProperty<BooleanProperty>(PropertyNames.Lossless).Value;
 
-            WebPFile.Save(input, output, quality, preset, scratchSurface, progressCallback);
+            WebPFile.Save(input, output, quality, preset, lossless, scratchSurface, progressCallback);
         }
     }
 }

src/WebPNative.cs

@@ -71,6 +71,8 @@ internal sealed class EncodeParams
             public float quality;
             [MarshalAs(UnmanagedType.I4)]
             public WebPPreset preset;
+            [MarshalAs(UnmanagedType.U1)]
+            public bool lossless;
         }
 
         [StructLayout(LayoutKind.Sequential)]