Fix several Dreamcast hardware regressions found while testing real PVR behavior.
- Upload the default white texture through the platform texture loader instead of memcpy directly into PVR texture memory.
- Add GPUTextureLoad so SH4 uses pvr_txr_load while the software backend keeps memcpy behavior.
- Track scissor/userclip application per polygon list and invalidate scissor state at buffer swap.
- Submit queued geometry before USERCLIP/header transitions in the SH4 submit path.
- Remove the stale packed 4bpp source-size double-halving in glTexImage2D.
- Add texture regression coverage for packed 4bpp, unpack row length, compressed palette4 forwarding, and 8bpp-to-4bpp packing.
- Update the scissor sample and add polygon_compare as a small hardware comparison sample.
Verified:
- make -C dcbuild gldc_tests scissor polygon_compare
- scissor sample on Dreamcast hardware
- polygon_compare sample on Dreamcast hardware
- existing strided texture sample on Dreamcast hardware
