[Misc] Add GetRenderImage() and fix DrawPixmap() on renderimages

[GWRon]

Requires bmx-ng/brl.mod#371

[Co-Authored by @davecamp aka Col]

[GWRon]

The PR also handles the following issues:

• Alt-Tabbing in a fullscreen application utilizing DirectX 9 (would not update backbuffer or renderimages but throw messages around) and proper handling of device loss/reset
• DrawPixmap outside of backbuffer regions and in render images in general (both also only issues for DirectX 9)

And it adds the ability to retrieve the currently set render image (if any)