SixteenBppTextureTest: test the FBO are renderable before drawing
SixteenBppTextureTest: test the FBO are renderable before drawing

It is valid for the implementation to return a
GL_FRAMEBUFFER_UNSUPPORTED for framebuffer completeness. Check for that
case and return early. This fixes the RGB5_A1 tests on Mac that doesn't
support rendering to this format.

BUG=angleproject:891

Change-Id: If37d8f82e994a93a470cbdc01d758fe60d54a5b2
Reviewed-on: https://chromium-review.googlesource.com/298863
Reviewed-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Tryjob-Request: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>
Tryjob-Request: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>
Tested-by: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>

MSOpenTech/angle,MSOpenTech/angle,mikolalysenko/angle,mlfarrell/angle,vvuk/angle,MSOpenTech/angle,mikolalysenko/angle,mybios/angle,ecoal95/angle,ecoal95/angle,mybios/angle,mlfarrell/angle,vvuk/angle,mybios/angle,ecoal95/angle,domokit/waterfall,vvuk/angle,ecoal95/angle,ppy/angle,ppy/angle,ppy/angle,vvuk/angle,domokit/waterfall,mlfarrell/angle,ppy/angle,mikolalysenko/angle,mybios/angle,ecoal95/angle,mlfarrell/angle,mikolalysenko/angle,MSOpenTech/angle