mesa: implement GL3 glClearBuffer() functions

mesa: implement GL3 glClearBuffer() functions

These functions clear color/depth/stencil buffers with a value that's
passed to the function, rather than the context clear values.

For now these functions are implemented in terms of the existing
ctx->Driver.Clear() hook.  In the future when we have non-normalized
integer and unsigned integer color buffer formats we'll need new
driver hook(s) to pass int and uint clear values to the driver.

Note: these functions are not hooked into the dispatch table at this time.