Detect the need of custom overlay

[spillerrec]

The FLIF api doesn't currently save transparent colors, making it impossible to use the overlay method for transparent images with this format, which is always used, even when not needed no transparency is needed. So FLIF support is uterly broken.
Furthermore, FLIF compresses better when we ignore transparent colors, so only saving them when needed could save us a little. This is likely also the case with WebP, though I haven't tried.

Work items:

• Raise issue at FLIF to find out how to save the transparent colors
• Detect the need of storing the transparent colors
• Support changing the settting in the encoder dynamically

[spillerrec]

Patch have been submitted and merged into FLIF for fixing the transparency issue.

I have made it possible to control this option in the Qt plugin by using the compression option, setting it to 1 instead of 0 removes alpha information. This is yet to be done with the WebP plugin.