Validate transform feedback conflicts on zero draw.
Validate transform feedback conflicts on zero draw.

This conflict should be validated even if the draw call would result in
no work being done on the GPU.

Bug: angleproject:2747
Change-Id: I6b32a07ff8a8598dd865270a3a935060f050a68f
Reviewed-on: https://chromium-review.googlesource.com/1162507
Commit-Queue: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Frank Henigman <4f3ccfa167f952ff7263448ff4377acff6a23e5c@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle