Adds a build option to use the capture libraries in the default output
location for libGLESv2. With |angle_with_capture_by_default| enabled,
libGLESv2 will have capture enabled and the non-capture libGLESv2 is
built as libGLESv2_no_capture.

Also moves the FrameCapture to be owned by gl::Context. Rearranges the
code a bit so that we don't require a separate version of libANGLE for
capture.

Also implements a read pixels entry point parameter.

Bug: angleproject:3611
Change-Id: Ic528e43e4779f794c8b6d0bf35410166dacc81b1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1719064Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>