Graphics output protocol cleanup

[HadrienG2]

This MR tries to improve the usability and safety of the Graphics Output Protocol by...

• Bringing documentation in sync with the UEFI spec's level of detail.
• Adding memory safety checks to the Blt function (if it is safe, it is not allowed to do buffer overflows)
  • We can later extract an unsafe back-end if that turns out to be a perf bottleneck for someone.
• Clarifying in which circumstances a BltBuffer stride must and mustn't be specified using an enum.
• Panicking if the framebuffer is accessed in a Blt-only mode
  • While this function is unsafe, I think the check is cheap enough (framebuffer is normally acquired once and used many times) and the error easy enough to make.
• Hiding the reserved byte of BltPixels (allowing the user to modify it is a type-safety risk).
• Homogeneizing the wording style of the data structure docs.

[GabrielMajeri]

LGTM. This PR reminded me to open #50