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.

V2: use 16bit formats; they're small enough that we don't run into
hardware limits on Gen6/Gen7, but closer to what was intended.

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