Webgl depth texture

- Enable the WEBGL_depth_texture extension on context init, see http://www.khronos.org/registry/webgl/extensions/WEBGL_depth_texture/

Note: Currently only Chrome implements this, Firefox doesn't support it. See https://bugzilla.mozilla.org/show_bug.cgi?id=783914 .

- Improve GL.getTexPixelData function so that it doesn't throw on WEBGL_depth_texture-related enums.