gles2: Add tests for EXT_clip_control

gles2: Add tests for EXT_clip_control

These teste are a port of gl4cClipControlTests.cpp/hpp to GLES2.

Apart from requiring EXT_clip_control the depth range tests also make
use of functionality from NV_read_depth.

v2: - Fix include protector
    - test NV_read_depth in init function instead of using the constructor
    - squash commits into one (alegal-arm)

v3: - Fix copy-paste error (Tapani Pälli)
    - Update license year

v4: Add new tests to Khronos test list (Alexandros Galazin)

v5: Correct GLES version in comment (Alexander Galazin)

v6: - Fix comment for header guard
    - Replace use of NV_read_depth by a resolve draw of the depth buffer
      to a RGBA surface (James Glanville)

Change-Id: Ib6610552798b5a48445f4e2d490c72421460d4cf
Signed-off-by: Gert Wollny <gert.wollny@collabora.com>