Use Egl instead of Glx

[costashatz]

This PR replaces WindowlessGlxAppilcation with the more powerful WindowlessEglApplication for Linux. Unfortunately, we are stuck with WindowlessCglApplication for Mac.

We also fix a small issue with OpenGL/GLFW3 detection..

[costashatz]

@jbmouret have a try on this one in your dockers! In essence, I replaced all glx with egl (see the ci/install_magnum.sh). If this works for you, I'll merge..

Now we can have windowless apps without dummy (or real) xservers! And in theory, GLFW should be better with egl!

[jbmouret]

I've run ci/install_magnum.sh then waf configure

The library compiles but not the examples.

[ 44/119] Linking build/pendulum
./libRobotDARTMagnum.a(cube_map_color.cpp.4.o): In function `CubeMapColor':
/home/pal/robot_dart/build/../src/robot_dart/gui/magnum/gs/cube_map_color.cpp:37: undefined reference to `Magnum::GL::AbstractShaderProgram::attachShaders(std::initializer_list<Corrade::Containers::Reference<Magnum::GL::Shader> >)'
./libRobotDARTMagnum.a(cube_map.cpp.4.o): In function `CubeMap':
/home/pal/robot_dart/build/../src/robot_dart/gui/magnum/gs/cube_map.cpp:36: undefined reference to `Magnum::GL::AbstractShaderProgram::attachShaders(std::initializer_list<Corrade::Containers::Reference<Magnum::GL::Shader> >)'
./libRobotDARTMagnum.a(shadow_map.cpp.4.o): In function `ShadowMap':
/home/pal/robot_dart/build/../src/robot_dart/gui/magnum/gs/shadow_map.cpp:32: undefined reference to `Magnum::GL::AbstractShaderProgram::attachShaders(std::initializer_list<Corrade::Containers::Reference<Magnum::GL::Shader> >)'
./libRobotDARTMagnum.a(shadow_map_color.cpp.4.o): In function `ShadowMapColor':
/home/pal/robot_dart/build/../src/robot_dart/gui/magnum/gs/shadow_map_color.cpp:32: undefined reference to `Magnum::GL::AbstractShaderProgram::attachShaders(std::initializer_list<Corrade::Containers::Reference<Magnum::GL::Shader> >)'
./libRobotDARTMagnum.a(phong_multi_light.cpp.4.o): In function `PhongMultiLight':
/home/pal/robot_dart/build/../src/robot_dart/gui/magnum/gs/phong_multi_light.cpp:39: undefined reference to `Magnum::GL::AbstractShaderProgram::attachShaders(std::initializer_list<Corrade::Containers::Reference<Magnum::GL::Shader> >)'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

[costashatz]

but not the examples.

This smells like a bad installation procedure... Maybe two magnums installed? Or some error that was ignored?

[jbmouret]

Yes, I will try again by removing all the current magnum...

[jbmouret]

So, it did compile in our docker. Should I try on mac too?

[costashatz]

For Mac, this shouldn't change anything. There's no egl in Mac. Only Cgl which was the one using already. Thanks for testing..!