big, ugly, error prone switch-statement is replaced with a compact table.
I also added numerous comments, including a comment explaining how the
format parameter is validated.
Explicitly pass GL_FLOAT as the type in the cases where that is the only
possible value (e.g., everywhere except the call to glColorPointer).
Validate that stride is >= 0.
Tested with all modes (including the two error modes) of
progs/tests/interleave.c.
Bug: #5001, #5058
Reviewed by: Brian Paul
5910dfacf1