ARB_texture_gather: add initial execution test

This test exercises combinations of:
- Shader stage vs/fs
- Offsetting
- Component count, up to implementation-defined limit
- Swizzling
- Unsigned normalized vs float type

The test works by numbering the texels, and ensuring that the four
texels sampled by textureGather*() are those we expect.

Not exercised yet:
- Integer textures
- Samplers other than sampler2D
- Other interesting internalformats [DEPTH_COMPONENT, etc]
- Addressing modes other than WRAP.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>