texelFetch/textureSize: use smaller formats

RGBA32I, RGBA32UI, and RGBA32F are huge and exceed some hardware limits
for multisample textures on i965
    - Gen6 can't do >64b per sample at all
    - Gen7 can't do 8X mode with >64b per sample

Use 16 bits per component instead, which still gives plenty of precision
for what the test is doing, but fits under the limits.

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