Write out padding regardless of length
- previous code wouldn't write out the padding for i=0
Fix transparent color bleed to quantized colors
- keep track of the normal nontransparent pixels, and only quantize those
- when finding color indexes for each pixel, ignore the quantizer's result and manually write out the transparent color
- getClosestPaletteIndex is gone: we can now directly store the transparent color's color table index in the Frame object, and don't need to scan through to find a closest match afterward.
Need to reserve a slot in the color table for the transparent color
- before, if we were excluding the transparent color from the quantizer's input, it may very well generate 256 actually used colors
- then there'd be no opportunity to write the transparent color to the color table
- with this change, limit the quantizer's output to 255 if we're planning on writing the transparent color as well