Followup to:
https://swiftshader-review.googlesource.com/c/SwiftShader/+/43950

Fixes dEQP.KHR_GLES31/core_compute_shader_copyimage.
This dEQP test was passing by chance, because load and store
were paired properly together, being equally incorrect with
regards to the OpenGL ES specs, which mentions:

GLES 3.1 spec, section 8.22:
If the individual texel identified for an image load or store
operation doesn’t exist, the access is treated as invalid.
Invalid image loads will return a vector where the value of
R, G, and B components is 0 and the value of the A component
is undefined. Invalid image stores will have no effect.

Bug: b/150464740
Change-Id: I5aa508ed7f08e234387478d3a6faee958ca9dd97
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/44868
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>