Add a test of alignment of transform feedback outputs.
Certain graphics hardware (notably Intel i965) has restrictions on unaligned memory accesses. However, transform feedback can be configured in such a way that data is not aligned on a natural boundary (e.g. a vec4 is 16 bytes wide, but is only required to be aligned to a multiple of 4 bytes). This test verifies that transform feedback data is written to memory correctly regardless of how the data is aligned. Reviewed-by: Ian Romanick <email@example.com>