A very incomplete proof of concept GPU renderer for Servo

After updating shaders in webrender, go to servo and:

1. Copy the webrender/res directory to servo/resources/shaders
2. Go to the servo directoy and do ./mach update-cargo -p webrender
3. Create a pull request to servo and update the shaders in the servo repository.

To use a custom webrender with servo, go to your servo build directory and:

1. Edit servo/.cargo/config - Create this file if it doesn't exist already.
2. Add 'paths = ["/Users/UserName/Path/To/webrender"]'
3. Build as normal