DRM source rectangle fixes

[6by9]

@cillian64 As discussed:

• fix for your sub 1 pixel source rectangle issue (even if we don't fully understand how it became <1pixel)
• potential patch to save LBM for your upscaled 1x1 solid colour fills. (I need to patch modetest or kmstest to allow me to test source rectangles easily)

[6by9]

Converted to draft as using the TPZ filter really doesn't work at present, and needs to either be fixed (if possible) or dropped.