primitive: Adds convenience constructors

primitive: Adds convenience constructors

This adds convenience primitive constructors named like:
  cogl_primitive_new_p3 or
  cogl_primitive_new_p3c4 or
  cogl_primitive_new_p3t2c4
where the letters correspond to the interleved vertex attributes layouts
such as CoglP3Vertex which is a struct with 3 float x,y,z members for
the [p]osition, or CoglP3T2C4Vertex which is a struct with 3 float x,y,z
members for the [p]osition, 2 float s,t members for the [t]exture
coordinates and 4 unsigned byte r,g,b,a members for the [c]olor.

The hope is that people will find these convenient enough to replace
cogl_polygon.