RenderingContext modernizations...

Files renamed and moved:
 - ParameterStructs ---> RenderingParameters
 - CoreRenderingData ---> internal/CoreRenderingStatus
 - RenderingData ---> internal/RenderingStatus

Features exported from RenderingContext:
 - internal/StatusHandler_glCompatibility added.
 - internal/StatusHandler_glCore added.
 - internal/StatusHandler_sgUniforms added.

RenderingContext: 
 - internally rename some members
 - Allow to specify the usage of a texture unit (refers to #12)
   - add additional usage parameter to texture functions
   - getTextureUsage(...) added.