TextureVk::copySubTextureImpl uses the format's pixelReadFunction and
pixelWriteFunction and was using the angle format (i.e. the format the
texture was created with) instead of the texture format (i.e. the actual
format used by the backend) to read and write pixels.  This was
specifically to make Luminance/Alpha formats work.  However, this was
incorrect for any emulated format.

This commit fixes the function to use the texture format.  To avoid
issues with Luminance/Alpha, this commit patches the pixel read/write
changes for these formats before using them.

Bug: angleproject:2913
Change-Id: I8981882b98502d869156a879bb7b5994943ecd8e
Reviewed-on: https://chromium-review.googlesource.com/c/1330261Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>