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.