d3d: Support offset rendering.
d3d: Flip v correctly when buffer swapping.
d3d: Flip viewport y within the top of the fb.

Fixes issues when the framebuffer is larger than the current render
target.
d3d: Converge DrawPixels() logic.

This makes render to offset actually work properly.
Merge pull request #6901 from unknownbrackets/gpu-convergence

Fix some issues with framebuffer sizing