Add content_offset for storing the device pixel offset in content
No longer accept a scale argument to get_tile_rects_page

In Servo layer bounds are now stored in layer/device coordinates and the
Servo compositor takes care of converting buffer requests to page
coordinates before passing them to the render task.
Merge pull request #90 from mrobinson/disentangle-scale

Allow rust-layers to stop caring about the Servo content scale
Set filter mode on tile textures to nearest when page scale is
one, and linear otherwise.
Merge pull request #91 from glennw/reftest-fix

Change texture creation to use nearest filtering. Fixes several reftests
Fix unused code warnings
Fix error checking bugs in load_shader.

This code was failing on Android because of an error flag set during an
earlier operation (`enable(TEXTURE_2D)`).  The logging was also broken because
get_error was called twice.
Fix compiler warnings on Android