This allows ANGLE to render directly onto a D3D swapchain in the correct
orientation when using the D3D11 renderer.

The trick is to add an extra uniform to each shader which takes either
the value +1.0 or -1.0. When rendering to a texture, ANGLE sets this
value to -1.0. When rendering to the default framebuffer, ANGLE sets
this value to +1.0. ANGLE multiplies vertex positions by this value in
the VS to invert rendering when appropriate. It also corrects other
state (e.g. viewport/scissor rect) and shader built-in values
(e.g. gl_FragCoord).

This saves a substantial amount of GPU time and lowers power
consumption. For example, the old method (where ANGLE renders all
content onto an offscreen texture, and then copies/inverts this onto the
swapchain at eglSwapBuffers() time) uses about 20% of the GPU each frame
on a Lumia 630.

Verification:
+ dEQP GL ES2 tests pass when "present path fast" is enabled
+ all ANGLE_end2end_tests pass when "present path fast" is enabled

BUG=angleproject:1219

Change-Id: Ib6eeea46bafa6ebce4adada0ae9db3a433b8fc4c
Reviewed-on: https://chromium-review.googlesource.com/321360Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tryjob-Request: Austin Kinross <aukinros@microsoft.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>