add glyphs in a spritesheet

[mk30]

this includes having to figure out how to include a custom spritesheet

[mk30]

it is now possible to include png images as sprites, so that different sprites are matched to different feature types.

screenshots:

[mk30]

bugs to investigate:

• png's without an alpha layer don't render properly
• error message when user tries to include non-png's in stylesheet
• png's with bw color profile may not work properly
• preserve sprite aspect value
• sprites may be horizontally flipped
• bugs if sprite block is wider than features block
• transparency is mostly fixed, but there are render order bugs: