If a clear becomes a no-op because the scissor has a width and height
of 0, return early and skip the draw.
This also works around a driver issue on some devices where it was
ignoring a null scissor and drawing the clear anyway. Found with deqp
test:
adb shell am start -n com.drawelements.deqp/android.app.NativeActivity \
    -e cmdLine '"deqp --deqp-case=dEQP-GLES2.functional.color_clear.* \
    --deqp-log-filename=/sdcard/dEQP-Log.qpa"'
run_angle_end2end_tests --gtest_filter=ClearTest.EmptyScissor/ES2_VULKAN

Bug: angleproject:3114
Change-Id: I6cf2716bd93bb332f74b44c7250e363c68cc614f
Reviewed-on: https://chromium-review.googlesource.com/c/1436841
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>