v0.3.7: Support OpenGL ES on *nix systems (#55)

* Support OpenGL ES on *nix systems

* support OpenGL ES on nix system (remove ES == mobile assumption)
  + add explicit options for both OGLES_GPGPU_OPENGL_ES{2,3}
    - these are mutually exclusive
    - if neither of the above is defined we use standard opengl (subset)
  + add OGLES_GPGPU_USE_EGL for EGL managed context (pass through to aglet)
* add generated header for OGLES_GPGPU_OPENGL_ES{2,3} version
  > w/ previous mobile == ES assumption this wasn't needed
  + update build with generated_file gl_definitions.h header to store defs
    > this wasn't needed in the exclusively mobile=es configuration
  + install generated gl_definitions.h header
  + introduce OGLES_GPGPU_NIX macro to indicate a nix variant
  + support includes of GLES2/gl2*.h, GLES3/gl3*.h, or GL/gl*.h headers as needed
  + update unit test to instantiate the correct context variant based on cmake options
* fix cmake logic in common_includes.h (android vs ios vs GL) for OGLES_GPGPU_OPENGL_ES{2,3} change
* video.h: use top level OGLES_GPGPU_TEXTURE_FORMAT, don't duplicate logic
* update aglet
  + bump version via URL + SHA1 in LOCAL config
  + translate OGLES_GPGPU_* -> AGLET_* options
* migrate test dependencies to unit test CMakeLists.txt
* update Hunter v0.23.151
* update HunterGate module
* bump version to v0.3.7

* update cmake package deps

* fix typo and cmake option location

* 's|ogesl_gpgpu|ogles_gpgpu'
* put required cmake options after project() and before hunter_add_package()

* fix opengl find_package() logic for *nix variants

* add include paths for mesa (test only) build