Make TextureOp use multitexturing to batch draws of different SkImages.

On a benchmark drawing small texture backed images this shows a 50% improvement on a Pixel XL 2016 and a little better than that on a Z840. 

Change-Id: I5c307ef0d8615aeae67bae7393874ad7e52eb233
Reviewed-on: https://skia-review.googlesource.com/42121
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
1 file changed