doc: texture documentation updates.

Remove duplicates, add minimal multisample docs, update GL function
mapping table.
doc: fix typo.

Embarrassingly enough it was copied to two another places.
doc: regroup gl*Draw*() commands in the table.

Group similar ones together (all the *Instanced ones will probably be
implemented using the same function).
Added EXT_shader_texture_lod ES2 extension.
Implemented texture LOD parameters.
Platform: display max lod bias in magnum-info.
doc: updated Sampler documentation.

Reference actual *Texture methods from there, it will be some time until
actual sampler objects are implemented.
Integer texture borders are not available in OpenGL ES.
Add forgotten check to extension-dependent texture tests.
Implementation of ARB_stencil_texturing.
doc: more GL command mapping updates.
Cleaned up duplicate stuff in CMakeLists.
Fix includes.

Vector3 was used but not included.
Queries for max supported texture size.
Platform: display max texture size in magnum-info.
Platform: nicer vector value display in magnum-info.
Doc++

Also remove redundant preprocessor branches.
Implemented depth texture compare mode and function.
Doc++
Doc++

Sometimes the specification hides the information very well.