Additional OpenGL ES fixes

Additional OpenGL ES fixes

- use highp int precision in fragment shaders (defaults to mediump)
- check for image uniform support in VTG stages
- get rid of extraneous image declaration
- use fewer SSBOs in the subgroup.basic.graphics tests
  and add in an explicit check for the number required (14)

Component: OpenGL
VK-GL-CTS Issue: 1698

Affects:
- KHR-Single-GL*.subgroups.*

Change-Id: I5c6aaa23be5fdd2eec82d567ae18310fc0054faa