insertRGBA processes regular tiles before primers, leading to confusing warning messages

[grunt-lucas]

If -Wcolor-precision-loss is enabled, and the warning triggers on colors that are present within both regular tiles and primer tiles, the warning text will imply that the regular tiles were processed before the primers. This is misleading to users, who will be under the impression that primer tiles are to be processed first. Let's fix that ordering so that the warning is consistent with the expected behavior.