Implement support for mixed sampled render targets

Adds a new FBO type kStencil_MSFBOType that is selected whenever
NV_framebuffer_mixed_samples extension is available.  In this new
FBO type a non-msaa color buffer is created with a multisampled
stencil buffer attachment.

Replaces numSamples() with separate numColorSamples and numStencilSamples
methods in RenderTarget.

In mixed samples mode non-MSAA codepaths are used to draw simple shapes,
while NVPR-rendered paths and text are rendered with a multisampled
stencil.

BUG=skia:3177

Review URL: https://codereview.chromium.org/1001503002

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