Make 420pack 'binding' layout qualifier tests check MAX_*_IMAGE_UNIFORMS

Make 420pack 'binding' layout qualifier tests check MAX_*_IMAGE_UNIFORMS

Image uniform support is not mandatory for the VS/TCS/TES/GS stages,
so GL_MAX_*_IMAGE_UNIFORMS can report 0.  On such an implementation,
these tests would fail.

This patch skips image testing in any such stages, making them
essentially passthrough shaders.

In BindingImageSingleTest, we also put the binding qualifier in the
fragment shader stage if the stage where we'd normally put it doesn't
support images.  This way, it's guaranteed to exist in at least one
shader stage.

Components: OpenGL

VK-GL-CTS issue: 712

Affects:
* KHR-GL45.shading_language_420pack.binding_images
* KHR-GL45.shading_language_420pack.binding_image_api_override
* KHR-GL45.shading_language_420pack.binding_image_array
* KHR-GL45.shading_language_420pack.binding_image_single
* KHR-GL45.shading_language_420pack.binding_image_default

Change-Id: Ic9eceb4472edff87d5de54ec6318cdb856b0bb8d