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.
Merge pull request #1868 from pelson/alpha_background

Fixed background colour of PNGs saved with a non-zero opacity.
Fix position of rotated text in Agg backend
Merge pull request #1968 from mdboom/rotated_text

Rotated text element misalignment in Agg