Add explicit type support for bitfieldReverse

[gnl21]

This was missing for 8- and 16-bit types.

[gnl21]

I tried to add a test for these functions to Test/ but I couldn't find any evidence that the glsl.* tests were actually running in the test suite, so it had no effect. Do the tests need adding to an explicit list? If so, where?

[dnovillo]

I tried to add a test for these functions to Test/ but I couldn't find any evidence that the glsl.* tests were actually running in the test suite, so it had no effect. Do the tests need adding to an explicit list? If so, where?

Perhaps look into how #3920 does it?