on application entry:
  there is a 1MiB - 8KiB area mapped writable and not
  executable with guard pages on either side, and rsp is
  set to the top of that. all other registers are set to 0.

for all system calls:
  rax, rdi, rsi, rdx are in/out paramters.
  rbx, rbp, rsp, r12-r15 are preserved.
  rcx, rflags, r8-r11 are clobbered.

encode color:
  rax in: 0
  edi in: r + g * 256 + b * 65536
  eax out: encoded color

get framebuffer:
  rax in: 1
  rax out: pointer to framebuffer
  rdi out: width + height * 2 ** 32
  esi out: pitch
  framebuffer is always 32 bpp. use the encode color syscall
  to encode colors. pitch is in dwords, not in bytes.