...by constructing ImmutableBitmap directly from SkImage.
This is a huge optimization for the case when content of canvas is
painted onto another canvas, as it allows pixels to remain in GPU memory
throughout the process.
Fixes performance regression on https://playbiolab.com/ introduced by
switching to GPU-backend for canvas.
By caching the SkImage that is reused across repaints, we allow Skia t
optimize GPU texture caching.
ImmutableBitmap is chosen to own the SkImage because it guarantees that
the underlying pixels cannot be modified. This is not the case for
Gfx::Bitmap, where invalidating the SkImage would be challenging since
it exposes pointers to underlying data through methods like scanline().
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.
