BMP: support RLE decompression (8bit/4bit)

[cfcs]

The compression methods seem to be documented at least somewhat here:

• 1 / 8-bit: http://www.fileformat.info/format/bmp/corion-rle8.htm
• 2 / 4-bit: http://www.fileformat.info/format/bmp/corion-rle8.htm

I see imagemagick using these compression methods, so maybe it would make sense to implement them if someone feels like it on a rainy day. :)

@@ -249,8 +251,10 @@ module BitmapMetaData = struct
     let compression_method_of_int n =
       match n with
       | 0 -> Ok RGB
+      | 1 -> Ok RLE_8bit
+      | 2 -> Ok RLE_4bit
       | 3 -> Ok Bitfields
-      | _ -> Error (`Bmp_error "Invalid compression method")
+      | u -> Error (`Bmp_error (Printf.sprintf "Invalid compression method %#x" u))