Dynamically generate image2D variables' declaration and function
definition in libANGLE.

Bug: angleproject:1987
TEST=angle_end2end_tests.ComputeShaderTest.*

Change-Id: Idacc756f7bd15f22eccb1d689e18e997f3e74159
Reviewed-on: https://chromium-review.googlesource.com/c/1142885
Commit-Queue: Xinghua Cao <xinghua.cao@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>