Digic X: Different RGB buffer resolutions and offsets.

[kitor]

At least on R5, GUI XCM buffer is no longer 960x540. In fact it is 2048x1080 with offset 152,30.

For now this is bodged in #62, but requires a better solution.

One of possible answers is to use XCM and draw on our own layer. This is blocked by #82, as if buffer is just malloc'd, XCM crashes with invalid IBus address.