clean up docs

new color scheme

add eraser tool

1. In the presence of nonzero shift_x / shift_y,
   stbtt_GetGlyphBitmapBoxSubpixel would return a nonzero-sized bounding
   box for empty glyphs (e.g. spaces). Since such glyphs don't have any
   outlines, the rasterizer wouldn't do anything, resulting in a 1x1-pixel
   image with uninitialized memory.
2. GetGlyphBitmapBoxSubpixel added shift_y then flipped the y axis,
   whereas the rasterizer flipped the y axis then added shift_y.
   Consistently flip-then-add in both places. This also makes the pattern
   of floors/ceils in GetGlyphBitmapBoxSubpixel simpler.
3. The rasterizer added shift_y after multiplying by the vertical
   oversampling factor, instead of before.

Vertical shifts now work much better, in my tests anyway.

switch srgb convert to second ryg method, which is a bit faster (8% total speedup on upsampling test);
remove extra table in slowpath

Make a guess at the value before doing the binary search, cuts the number of conditionals by half. Not as much gain as I hoped but something.

reformat docs;
new simple uint32 test

Resample