Remove pre-allocated clip space from GrRenderTargetOpList

It seems like this should be safe (perf-regression-wise) since the arena allocated clips are used infrequently.

In the following, each GM run has 621 srcs and a total of 81087 ops.
Each skp run has 87 srcs and a total of 14709 ops.
The following table shows how many times the arena allocator is called for each config/src.
Overall, I think is safe to only allocate the arena-allocated space on an as needed basis.

gl-gm:       0
gl-skp:      0

gl-msaa4:    0
gl-msaa4:    0

glinst-gm:   1225
glinst-skp:  579

glnvpr4-gm:  623
glnvpr4-skp: 77

Change-Id: I78e04b060b6ad3b78697519249bb4b9c3193fc85
Reviewed-on: https://skia-review.googlesource.com/14392
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
2 files changed