LibGfx+LibWeb: Add ImmutableBitmap for images bitmap caching in painter

Before this change, we used Gfx::Bitmap to represent both decoded
images that are not going to be mutated and bitmaps corresponding
to canvases that could be mutated.

This change introduces a wrapper for bitmaps that are not going to be
mutated, so the painter could do caching: texture caching in the case
of GPU painter and potentially scaled bitmap caching in the case of CPU
painter.

Userland/Libraries/LibGfx/Forward.h

@@ -9,6 +9,7 @@
 namespace Gfx {
 
 class Bitmap;
+class ImmutableBitmap;
 class CharacterBitmap;
 class Color;