memset uses rep stosb on x86 during boot, with memory
not set to write-combining, which makes it slow.
Instead we do aligned writes of 2 x four bytes at once.
Only clear the minimum of size and width * height * 4
UEFI framebuffer size can be huge, upto 512MB here,
and rep stosb seems to be around 25-30MB/s
This is written as generic as possible to work on
old compilers and different platforms, without
expecting boot memset to be optimized.
This makes it almost unnoticable compared to not
clearing.
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