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/CMakeLists.txt

@@ -60,6 +60,7 @@ set(SOURCES
     ImageFormats/WebPLoader.cpp
     ImageFormats/WebPLoaderLossless.cpp
     ImageFormats/WebPLoaderLossy.cpp
+    ImmutableBitmap.cpp
     Painter.cpp
     Palette.cpp
     Path.cpp