update SDL_render_d3d12.c after DPI scaling/highdpi support patch.

[sezero]

See #5778 (comment)

(CC: @chalonverse, @ericwa)

[chalonverse]

At least on the d3d12 side of things, the renderer still seems to work fine. Not entirely sure how to test the highdpi support, though.

[ericwa]

For testing the highdpi support, you'd:

• set the SDL_WINDOWS_DPI_SCALING environment variable to 1 (or set the hint with SDL_SetHint)
• launch a test app (I like to use SDL's testwm2)
• change the Windows scale factor (right click on the desktop -> display settings -> scale and layout heading -> change the scale combobox e.g. from 100% to 125%)

The renderer surface size should change e.g. from 640x480 to 800x600 pixels, but the SDL window size should remain at 640x480 points.

[sezero]

Any results? Decision?

[chalonverse]

I tried it out just now -- with that hint enabled and 150% scaling (which is what I use anyway), the window remains 640x480 but the viewport resolution is now 960x720. So, it seems to work as expected.