Do not render a zero texture

If a texture has not yet been initialized do not render it. This can
happen if a BufferRequest has not yet been fulfilled.
Merge pull request #88 from mrobinson/render-zero

Do not render a zero texture
Abstract away more details of shaders into structs

This allows RenderContext to not care about more details of programs in
preparation for supporting more varieties.
Add support for showing debug borders

These borders show the edges of layers and tiles, to aid in debugging
the layer tree.
Merge pull request #89 from mrobinson/debug-borders

Cleanup shaders in rendergl.rs and add support for debugging borders
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
Remove unused code on Linux
Merge pull request #93 from mbrubeck/unused

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
Merge pull request #92 from mbrubeck/android-err

Android build and crash fixes
Fix use-after-free on Android.

Fixes servo/servo#2916 (broken rendering on Android), which happened because
the `bitmap` pointer outlived the Vec<u8> that it pointed to.
Make NativeSurface::upload a mut method
Replace transmute with as and as_mut_slice
Merge pull request #95 from mbrubeck/android-bitmap

Fix use-after-free on Android.