Add color space fallback for matrix convolution filter

Add color space fallback for matrix convolution filter

GrMatrixConvolutionFilter is used in two places right now.
The GaussianBlur code ensures that we blur in the space of
the source image, so no conversion is needed. This changes
SkMatrixConvolutionImageFilter to give the same guarantee.

BUG=skia:

Change-Id: I1a6199d4893f2e579dc6fa809c59cd195bd62f90
Reviewed-on: https://skia-review.googlesource.com/6467
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>