This function highlights a region of text in the buffer.
The range between start and end will be highlighted with the color in color, and bitmap will be painted over the range of text in black and white. If bitmap is #f, the range will be inverted, using the platform specific xor. This method is not reccommend, because the selection is also displayed using xor.
The function will return a thunk, which when invoked will turn off the highlighting from this range.