arb_texture_multisample: new test for teximage state

This tests that the new teximage state added in ARB_texture_multisample
exists and has correct defaults for non-multisample textures:

    - GL_TEXTURE_SAMPLES = 0
    - GL_TEXTURE_FIXED_SAMPLE_LOCATIONS = false

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
arb_texture_multisample: new tests for sample mask

Adds tests for sample mask:

- Initial state
- Fetching of each sample mask word
- Execution test via rendering with various masks to a multisample FBO,
  then downsampling via glBlitFramebuffer and checking the result.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
arb_texture_multisample: add new sampler targets to textureSize

Covers use of the textureSize() glsl builtin with multisample samplers

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
arb_texture_multisample: add test for texelFetch()

Writes a test pattern to individual samples in a multisample texture,
and tests texelFetch() reads the correct values for each sample.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
rude hack: use smaller formats in texturing tests.

RGBA32I, RGBA32UI, and RGBA32F are huge and
unsupported for multisample textures on at least Gen6 i965.

This shouldn't be committed as-is, but it allowed for testing of
multisample samplers with the generic textureSize() test.

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