[cogl] Adds a bitfield argument to cogl_clear for specifying which buffers to clear

[cogl] Adds a bitfield argument to cogl_clear for specifying which buffers to clear

Redundant clearing of depth and stencil buffers every render can be very
expensive, so cogl now gives control over which auxiliary buffers are
cleared.

Note: For now clutter continues to clear the color, depth and stencil buffer
each paint.