Add a parameter GR_ALWAYS_ALLOCATE_ON_HEAP to allow for only ever creating temporary buffers on the heap instead of allowing stack allocation

Unfortunately this can't be a GR_GL_* config like we'd discussed because there are SkAutoSMalloc buffers that are built outside of gl/

R=bsalomon@google.com, bsalomon
BUG=skia:

Author: george@mozilla.com

Review URL: https://codereview.chromium.org/459263003
5 files changed