Fix #1949: Be explicit about needing Gtk 3.0 when importing gi so we can provide a better error message
Add gtk3cairo, gtk3agg tests to template
Merge pull request #1949 from mdboom/setup/gi-explicit-version

Build failes under ubuntu 13.04
Fixed background colour of PNGs saved with a non-zero opacity.
Switch agg backend to use pixfmt_rgba32_plain instead of pixfmt_rgba32, which fixes a problem with alpha-blending partially transparent backgrounds.