Like the GL backend, use the CPU copy path rather than the draw path
when the target is sRGB.

Fixes the following WebGL tests:
conformance2/textures/canvas/tex-2d-srgb8_alpha8-rgba-unsigned_byte.html
conformance2/textures/canvas/tex-2d-srgb8-rgb-unsigned_byte.html
conformance2/textures/image_bitmap_from_canvas/tex-2d-srgb8_alpha8-rgba-unsigned_byte.html
conformance2/textures/image_bitmap_from_canvas/tex-2d-srgb8-rgb-unsigned_byte.html

Re-enable CopyTextureTestES3.ES3UnormFormats for Vulkan as they now pass
with this change.

Bug: b/157934810
Bug: angleproject:4092
Change-Id: I6b76e5f4345d204a084f5483e206c5bb4ff2f139
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2233405Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>