Windows: Improve EVR presenter.

Removed hard dependency to Angle; it's now required only when rendering
to a texture. If Angle is not used, the buffers are now mappable to
system memory.

This enables HW-accelerated video decoding in QML for non-ANGLE
builds. Note that the frame data have to do a round-trip through system
memory, which is not ideal but still faster than SW decoding.

Task-number: QTBUG-45593
Change-Id: Icc3503142df4e8dbb53b4e11f409b161fd2f9bde
Reviewed-by: Christian Stromme <christian.stromme@theqtcompany.com>