HLSL allows image and texture types to be templatized on sub-vec4 types,
or even structures. This was mostly handled already during creation of
sampling operations. However, for operator[] which can generate image
loads, this wasn't happening.
It also isn't very easy to do at that point in time, because operator[]
does not know where the results it produces will end up. They may be
an lvalue or an rvalue, and there's a post-process to convert loads to
stores. They may end up in atomic ops.
To bypass that difficulty, GlslangToSpv now looks for this case and
adds the appropriate conversion. LIMITATION: this only works for
cases for which a simple conversion opcode suffices. That is to say,
it will not work if the type is templatized on a struct.
