Save fix

[syockit]

Minor papercut fixes for save image & save image sequence.

• Image sequence can be saved as PNG (though it might be unwise to do so)
• Save Image dialog now uses file type filters, so if you forget to put the extension, it will automatically add it for you according to the selected filter. Dialog opens with the filter selected according to the one in preferences.

[syockit]

Oh, forgot to mention that I fixed the save routine so that the image type is inferenced from the file extension. I'm assuming that there's no path to messing that up, as the "Save Image" dialog automatically adds the extension according to selected filter. Wait, let me check…

…oops, there seems a way to abuse it. If filename contains non-image registered extension, it will refuse to save. If filename is an image extension that Qt recognizes, it will be saved using that format instead (managed to do it with bmp and tif). I suppose I need to revert that particular change.

[grahamrow]

Well either way it's better than the current version (which will save files with the wrong suffix).

I'm actually planning to work on a feature to automatically save to movies directly, though I can't decide whether to simply call shell commands on some temporary files or actually use the ffmpeg API directly (the latter being more foreboding). Thoughts?

[syockit]

While the prospect of being able to feed frames directly to the encoder without temporary files sounds tempting, having extra dependency might be a headache. Or are you going to embed the whole library (ffmpeg or libav?) in the project?

[syockit]

I'm sorry, I couldn't find way to workaround this without overriding the file browser. I think I'll come up with a error popup when the image fails to save, by checking if the filename it tries to save to is not created. (Of course, the caveat is the popup won't appear if there's already a file by the same name e.g. "image.txt")