Add offset to xcursor

This patch adds the option to specify an offset for the cursor position.

Small fix to texture format aswell.
Add support for multi-screen setups

This patch adds support for multi-screen setups by using xinerama
information if available to determine the screen size and position.

In case xinerama is not available or not active it uses the information
provided for the x screen.

To clean up the code some generic helper functions were moved to a
separate source file.
Add README for the xshm plugin

The file contains some development information specific to the plugin.