loader: framebuffer should only be written into

[tsoome]

Summary

Reading from Write Combining memory can be very-very slow. Try to use
shadow buffer to avoid such reads.

Test Plan

Both UEFI and VBE frame buffers appear to behave as expected, no
visual defects. My ability for performance tests is rather limited,
unfortunately.

[val_packett.cool]

Excellent, this very significantly improves the loader's performance on my desktop (AMD Radeon RX Vega GPU)