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
dded69693d