This adds a "--screenshot-dir" argument to capture screenshots.
If we're running with screenshot capture then the test will
early exit after the first capture. The screenshots use the same
naming pattern as the test name:

TracePerfTest.Run/vulkan_trex_200 -> angle_vulkan_trex_200.png

Note the screenshot dir is relative to the test binary directory,
not the CWD. Also adds a PNG saving utility function.

Bug: angleproject:4615
Change-Id: I1de8ae6a6e6892586bb0b743e7b9a842f90f98e5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2184834
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>