Implement support for more than one WebGL graphics mode config.

[juj]

At the moment the implementation of EGLConfig structures in EGL supports a single hardcoded config. Add support for different combinations of rgb+alpha+depth+stencil+msaa as possible in http://www.khronos.org/registry/webgl/specs/1.0/#WEBGLCONTEXTATTRIBUTES .

Investigate how to best expose premultipliedAlpha and preserveDrawingBuffer booleans through EGL.

[stale]

This issue has been automatically marked as stale because there has been no activity in the past 2 years. It will be closed automatically if no further activity occurs in the next 7 days. Feel free to re-open at any time if this issue is still relevant.