video_core: Generate mipmap texture by drawing

[vonchenplus]

It seems that some games use drawing to generate mipmap textures, and the emulator does not currently support this behaviour very well.

When this behaviour is triggered, the viewport transform is disabled, so we can set the current viewport to the drawing area and rescale the vertex position.

To

Fixed： Icons in RetroArch

DQB sees similar behavior, but needs more fixes.

Warning: This will broken the existing shader cache.

[Timmy5942]

Rebase?