gallium/draw: apply DRAW_PIPE_FLAG_MASK to all vertex elements

Depending on first/last provoking vertex either the first or last
element/index may have the extra DRAW_PIPE_x flags.  Mask off those
bits for all vertex elements to be safe (esp. for the quad case).
This may be overly cautious, but it's a cheap operation.

Fixes a segfault caused by large/bogus vertex indexes otherwise.
1 file changed