[graphite] Add drawContext pointer to task dumping

* When SK_DUMP_TASKS is enabled, drawTasks will record the pointer to the drawContext which created the task.

* DrawTask prefix lengths are not always a multiple of four, so remove an assert that was a little to aggressive.

Change-Id: I2eff4464e176505bb4d956bc1081b55882590888
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1065156
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Thomas Smith <thomsmit@google.com>