GraphicsContext:drawImage() with blend mode doesn't work (or if we load an indexed image)

[dacap]

From: https://community.aseprite.org/t/unable-to-draw-to-transparent-image-to-dlg-canvas/19406/3

We cannot use the GC blendMode correctly with the GraphicsContext:drawImage(image, x, y) function (the srcRect, dstRect version does work).

Also we should be able to draw indexed images with transparent pixels.