Determine the destination internalFormat correctly.

dEQP-EGL.functional.image.modify.renderbuffer_rgb565_tex_subimage_rgb8
dEQP-EGL.functional.image.modify.renderbuffer_rgba4_tex_subimage_rgba8
dEQP-EGL.functional.image.modify.tex_rgb565_tex_subimage_rgb8
dEQP-EGL.functional.image.modify.tex_rgba4_tex_subimage_rgba8
dEQP-EGL.functional.image.modify.tex_rgba8_tex_subimage_rgba5_a1
dEQP-EGL.functional.image.modify.tex_rgba8_tex_subimage_rgba4
dEQP-EGL.functional.image.modify.tex_rgba5_a1_tex_subimage_rgba8
dEQP-EGL.functional.image.modify.tex_rgba5_a1_tex_subimage_rgba4
dEQP-EGL.functional.image.modify.tex_rgba4_tex_subimage_rgba8
dEQP-EGL.functional.image.modify.tex_rgba4_tex_subimage_rgba5_a1
dEQP-EGL.functional.image.modify.renderbuffer_rgba4_tex_subimage_rgba8
dEQP-EGL.functional.image.modify.renderbuffer_rgba4_tex_subimage_rgba5_a1
dEQP-EGL.functional.image.modify.renderbuffer_rgb5_a1_tex_subimage_rgba8
dEQP-EGL.functional.image.modify.renderbuffer_rgb5_a1_tex_subimage_rgba4

Bug: angleproject:3170
Change-Id: Ic9870390b2d4a0dcbe561efd3fb1597faadb7d79
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1524404
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>