Introduces a few ways of controlling capture via:

 * `ANGLE_CAPTURE_ENABLED`:
   Can be set to "0" to disable capture entirely.
 * `ANGLE_CAPTURE_OUT_DIR`:
   Can specify an alternate directory than the CWD.
 * `ANGLE_CAPTURE_FRAME_END`:
   This variable can override the default of 10 frame to capture.

Also adds a simple usage guide document.

Bug: angleproject:3611
Change-Id: I49d32d5bae2c490dcbeb0f1ce3c6038e433adfaa
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1869542Reviewed-by: Tobin Ehlis <tobine@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>