If transform feedback is active, primitives still need to be rendered even
if no pixels are written to the framebuffer.

Instead of checking for active transform feedback, simply remove the draw
call skipping optimization since it is most likely an application mistake to
draw with a zero sized viewport and we shouldn't optimize for this case.

This change doesn't affect the clear calls because the viewport is set to the
framebuffer size which is non-zero.

BUG=angle:743

Change-Id: I04af9d6de5aad3040e3c6b3c24990e107e21ad36
Reviewed-on: https://chromium-review.googlesource.com/218508Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>