Ability to have non-unique named shader resources

Created by: Xottab-DUTY

Currently, R_constant_table class located at Layers/xrRender/r_constants.h requires unique named ref_constant objects. We need to support ability to store different objects with the same name. Refactored code should not be slower than existing.

I suppose, that the main problem is in method get that searches only by name.

Suggestion: Add a third overload for method get with two arguments – name and type

Any other suggestions are welcome.

P.S. This is required for DirectX 11 Feature Level 9 support.