Remove unnecessary dirty clip flag in GrGpu, remove getUsableStencilBits()

getUsableStencilBits() isn't being called anywhere. It will problematic when stencil buffers aren't statically bound to render targets but are bound lazily when needed.

If some code path really needs this we'll have to figure out a new semantic for allowing the client to bind a SB before drawing and release it afterwards.

[reed1, 2011-08-01 15:25:13]

SWE -- sure, whatever

[bsalomon, 2011-08-01 16:53:13]

Closed with the timely r2011.