Optimisation to Renderer

Release 0.12.3

STPHeightfieldTerrain

• Addresses issue #35 by computing terrain mesh using double precision before casting to single precision and storing results.
• Introduce STPPlaneGeometry as a plane generator that allows speficiation of number of subdivision on a plane and optimised for fast and high-precision generation of high quality mesh.
• Refactor terrain shader to allow making use of the new plane generator and avoid precision problem.
• Simplify terrain texture splat region lookup using shader pointer. Remove registry dictionary from the terrain fragment shader, now region registry holds uses pointers to splat texture and null pointer to identify regions with no texture data.

General fixes and improvement

• Introduce STPBindlessBuffer as a thin wrapper over buffer address. This requires extension GL_NV_shader_buffer_load, however.
• Remove extension check in STPBindlessTexture because this is mandatory anyway.
• Refactor academic report reference to a single file.
• Add a new GLSL common header STPNullPointer.glsl to help better interacting with use of pointers in shading language.
• STPLightSpectrum now manages its own texture handle instead of keeping an array of spectrum handle in STPScenePipeline.
• Add documentation for texture definition language.