* implemented instanced rendering (note that this is "always on" and doesn't require an additional kernel compilation -> in cases where no instancing is used, the instance_id will simply be 0)
 * renamed most (if not all) occurences of "triangle" to "primitive" (since other primitives than triangles might be rendered)
 * user transform_programs now have access to the "built-in" variable "instance_index"
 * user rasterization_programs now have access to the "built-in" variables "primitive_index" and "instance_index"
 * oclraster_support: now that instanced rendering is working, implemented font rendering (+fixed blending, +use a RGBA UINT_8 texture instead of a RGB one, so it works with native image support)
 * some cleanup
 * updated README (new 2D ui + 3D screenshot)