Environment rendering pass

[cgphilipp]

This adds an EnvironmentRenderingPass, which renders four types of environment textures types via an ndc triangle.
includes #244, merge after that one
closes #114

[coveralls]

Coverage decreased (-0.3%) to 17.614% when pulling f01217a on p-otto:env-mapping-pass into 40a4968 on cginternals:master.

[cgphilipp]

@cgcostume While working on this, I found out that we are handling cubemap faces in an incorrect way. The +x, -x, +z and -z faces have an inverted Y coordinate in comparison to the usual WebGL way of handling texture mapping: https://www.khronos.org/opengl/wiki/Cubemap_Texture#Upload_and_orientation

Currently we are implicitly flipping all faces in TextureCube::fetch, however this results in the wrong orientation in the X axis. To be correct, we would need to instead rotate all faces by 180°.
This is currently worked around in the envmapping example with a ray.x *= -1.0, but when implementing reflections using cubemaps this workaround is kind of counter intuitive.

One way to fix this is to perform the 180° rotation either in ERMT or during loading cubemaps in webgl-operate. How should we proceed here?

[cgcostume]

• ermt should display the textures as it is implemented now,so -x +z +x -z should always be visible like this.
• ermt should export the textures as described by khronos
• webgl-operate should import as is, since it's assumed to be already correct
• our shaders used for env mapping should be adjusted/fixed accordingly

[cgcostume]

@p-otto what's your opinion on this?

[cgphilipp]

@cgcostume Sounds good, maybe we can later add an export option for ERMT to target either WebGL or other tools/APIs that use the conventional format.