This fixes WebGL tests on Vulkan (e.g. SwiftShaderVK) that copy from the
canvas (GL_RGBA8) to an integer format target texture, such as:
conformance2/textures/canvas_sub_rectangle/tex-2d-rg8ui-rg_integer-unsigned_byte.html
The sampled normalized value needs to be scaled up by 255.

Also fixes angle_end2end test CopyTextureTestES3.ES3UintFormats on
Vulkan.

Bug: b/156399229
Bug: angleproject:4092
Change-Id: I18d46bc4453fdc500caca96ca3a12fe93290b10c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2219142Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>