Fixed formatting and dispose handling in GIF decoder

Part of the GIF decoder was ported incorrectly to ActionScript, which led to incorrect decoding of optimized GIFs (#66). This fixes the (existing) decoder code without breaking for GIFs with dispose-values of 2 or 3 like #181.

Also fixed formatting to use the same conventions as the rest of the source code, which is why the diff is messy. Whitespace-free one: https://github.com/LLK/scratch-flash/pull/190/files?w=1