We have a per-thread pointer to select which api struct is used, and GL functions are pound-defined to call the versions in that struct.
GL/GLES surfaces can use one of the loader function to load their api, and the prepare proc of the surface sets its api as the current one
before gl calls are issued.
- Simplified shader names, prefix embedded strings by "glsl_"
- Extracted shader compilation in common function
- Add shader version string and common structs in canvas code rather than in every shader file
- Renamed zIndex with shapeIndex everywhere
Martin Fouilleul