CI: Validate Shaders

[jonas-schievink]

We've had many issues where the nVidia driver would accept our shaders while other drivers (mainly Intel) would reject it (rightfully so). We should use the reference compiler (https://github.com/KhronosGroup/glslang) to validate our shaders with Travis instead.

Unfortunately I haven't yet found an easy way to install the compiler in our Travis environment (without using apt-get), so we might lose the fast builds of the container-based infrastructure.