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Function : Layer_Freetype::new_font_
Explicitly require Python version 3 for plugin scripts and automatically choose suitable binary.

Python binary could be named as "python" or "python3" - Synfig checks the version at runtime and chooses suitable binary.
When set a Cairo surface it is needed to map it i order to be used by the importer
Fig bug #418 - Image isn't displayed if it's grouped (Cairo)
Layer Paste Canvas now passes the render method to its canvas.
Fix bug #422: Crash: map_cairo_image(): Assertion failed 

There weren't any checking of the status of the cairo_surface_t when reading the image from file. This commit adds it and fixes the bug.
Bug report #420 - The Add keyframe button shouldn't be active if there is not timeline
Bump ETL to 0.4.16, synfig and synfigstudio to 0.64.0 and add splash screen for 0.64.0
Update version in releases and app
Update NEWS