When copying via glTexSubImage2D to a non-renderable format, GL_RGB9_E5,
ANGLE's TextureVk::copySubTextureImpl will use the read-back and copy
method. This path was ignoring the source offset in the source area
computation.

Fixes the following WebGL tests:
conformance2/textures/canvas_sub_rectangle/tex-2d-rgb9_e5-rgb-float.html
conformance2/textures/canvas_sub_rectangle/tex-2d-rgb9_e5-rgb-half_float.html

Bug: b/157744725
Change-Id: I714f8d3b8f1490edab5e7578445e9623215ce229
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2225611Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>