Add a few RGB-aware set & blend methods to the blitters (#1680)

* BlitBuffer: Added a fill_rect variant that takes an RGB32 color as input
* BlitBuffer: Added a couple of blend OVER & blend MUL methods to handle color highlights
* BlitBuffer: Added a color_blit_from variant that takes an RGB32 color as input
* C BlitBuffer: Fix a few minor compiler warnings
* BlitBuffer: Rename the awfully named dimRect & lightenRect methods to lightenRect & darkenRect, respectively. Because they did the opposite of what it said on the tin. Added more comments explaining the results to avoid further confusion.
* BlitBuffer: Defined a set of highlight colors as `BB.HIGHLIGHT_COLORS` (user patchable), most of them having a more pastel variant because those look slightly less terrible on Kaleido panels.
* BlitBuffer: Added a few miscellaneous methods to ease working with this in front.
* Kobo MTK: Increased the alignment constraint, because the HL color popup was getting cut off.
* MuPDF: Support color annotations in `addMarkupAnnotation`

Many thanks to @smasher816 for the initial implementation ;).