Skip impossible shader stages in image load store max uniforms test.

Skip impossible shader stages in image load store max uniforms test.

Implementations not required to support images in the geometry stages,
and are allowed to advertise 0 for:

    * GL_MAX_VERTEX_IMAGE_UNIFORMS
    * GL_MAX_TESS_CONTROL_IMAGE_UNIFORMS
    * GL_MAX_TESS_EVALUATION_IMAGE_UNIFORMS
    * GL_MAX_GEOMETRY_IMAGE_UNIFORMS

This means that gl_Max*ImageUniforms may be 0, which means that

    #define N_UNIFORMS gl_MaxVertexImageUniforms
    layout(r32i) uniform iimage2D u_image[N_UNIFORMS];

will define a zero sized array, which is illegal.  To work around this,
simply skip stages that have zero supported image uniforms - there's
nothing to test there, anyway.

Components: OpenGL

VK-GL-CTS issue: 712

Affects:
* KHR-GL45.shader_image_load_store.uniform-limits

Change-Id: I14116ff8b4714e04b35ce0929ed68ed0f38e7d52