Add a test that creates a framebuffer formatted GL_DEPTH_COMPONENT24 and
read pixels from it using glReadPixels.

This test uses a combination of the GL_OES_depth24 and GL_NV_read_depth
extensions.

This test can only run on GLES3 as the GL_DEPTH_COMPONENT24 internal
format is not available on GLES2.

Test: angle_end2end_tests --gtest_filter=DepthStencilTestES3.ReadPixelsDepth24/ES3_Vulkan_SwiftShader
Bug: angleproject:5799
Change-Id: I4e898bf756498796df02ef41cc2b989df78f13df
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2817765
Commit-Queue: Lubosz Sarnecki <lubosz.sarnecki@collabora.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>