Shader System rework

[Haeri]

The current shader system needs a rework. It should not be necessary to create a class for every shader type. A shader instance should parse all the necessary information from a unified shader file (with a .pbs extension) containing vertex and fragment source code. Additional defines should specify how the shader is configured.

shader.pbs

# type: surface
# vertex
/* glsl code */

# fragment
/* glsl code */