Blur can now use A8/R8 render targets

Updated blur to use A8/R8 render targets when available. This is only a performance win for large blurs (> ~16)

[bsalomon, 2012-04-10 13:35:45]

How about this model: GrGpu has a boolean table with one entry per GrPixelConfig
filled out by the subclass's constructor. It has member isConfigRenderable() and
GrContext has a passthough isConfigRenderable() that calls the gpu's function.
Not too long ago SkGpuDevice stopped including GrGpu. I'd like the files in the
skgr target not to include from src/gpu and remove GrContext::getGpu(). If it's
a lot of work to figure out whether all the other formats are renderable then we
can punt on having a full table but I'd still prefer not to expose SkGpuDevice
to GrGpu.

http://codereview.appspot.com/5988071/diff/1/src/gpu/gl/GrGpuGL.cpp
File src/gpu/gl/GrGpuGL.cpp (right):

http://codereview.appspot.com/5988071/diff/1/src/gpu/gl/GrGpuGL.cpp#newcode377
src/gpu/gl/GrGpuGL.cpp:377: // OpenGL 3.0   : supports both A8 and R8 as color
render targets
GL_ARB_framebuffer_object (which only requires GL 1.1) makes ALPHA renderable.
GL_EXT_framebuffer_object does not.

[robertphillips, 2012-04-10 18:56:02]

New version

[bsalomon, 2012-04-10 19:15:00]

On 2012/04/10 18:56:02, robertphillips wrote:
> New version

LGTM modulo adding comment about RGBA4 always being available b/c we require
either the EXT or ARB FBO extension to be available.

[robertphillips, 2012-04-10 19:27:04]

committed as r3647