vdr-plugin-text2skin

Changes:

- Fixed some compiler warnings.
- Makefile adjustments for VDR 1.7.13.
- Fixed segmentation fault with 16bit images (closes #258 - thanks to
  domml@vdrportal)
- Fixed crash with GraphicsMagick >= 1.3.8 and non-XPM images.
- Fixed crash when deleting ongoing timers or their recordings (closes #306)