Use the blitter to implement Image::clear.

The image can be easily packed into a sw::Surface. For now,
the surface is a temporary, but it may become a member in
the future if it makes sense.

Bug b/119620767

Change-Id: I122a321509dc8cc5b624ddd776bd9059eee0cbc4
Reviewed-on: https://swiftshader-review.googlesource.com/c/23109Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>