Implement approx-match support in image filter saveLayer() offscreen.

Implement approx-match support in image filter saveLayer() offscreen.

Currently, the GPU-side image filter implementation creates exact-match
textures for the offscreen backing stores for saveLayer().  This is
because several filters have GPU implementations which depend on the
texture coordinates being 0..1.

The fix is three-fold:

1) Store the actual requested size in the SkGpuDevice, so that when
wrapping it in an SkBitmap for passing to filterImage(), we can give
it the original size.
2) Fix the filters (SkMagnifierImageFilter, more TBD) whose GPU
implementation depends on 0..1 texture coordinates.
3) Remove the exception for GPU-side image filters in
SkCanvas::internalSaveLayer().

N.B.: this change will cause some minor pixel diffs in the
GPU results of the following GMs (and possibly more):
matriximagefilter, matrixconvolution, imagefiltersscaled,
lighting, imagemagnifier, filterfastbounds,
complexclip_aa_Layer_invert, complexclip_aa_layer,
complexclip_bw_layer_invert, complexclip_bw_layer.

BUG=skia:3532

Review URL: https://codereview.chromium.org/1034733002