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This makes the testapp rendering code more closely match vr_shell_gl,
alleviating the annoying hover issue (a button may stay hovered even
when the reticle had moved away).

BUG=
R=ymalik

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_vr;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I6b1bd7006c9ff319c177d721a309d82906a69153
Reviewed-on: https://chromium-review.googlesource.com/998740Reviewed-by: Yash Malik <ymalik@chromium.org>
Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548761}