Primitive constants

The following constants are used with draw_primitive_begin(kind).
 * pr_pointlist: The vertices are a set of points.
 * pr_linelist: The vertices are pairs of line segments. Each pair makes one line, so there must be an even number of vertices.
 * pr_linestrip: The vertices form a constant set of lines. Each vertex connects to the next. The last vertex is not automatically connected to the first.
 * pr_trianglelist: The vertices are sets of triangles. The number of vertices should be a multiple of 3.
 * pr_trianglestrip: The vertices again form triangles but this time it works slightly different. The first three form the first triangle. The last two of these vertices, together with the next vertex, form the second triangle, etc. So each new vertex specifies a new triangle connected to the previous one.
 * pr_trianglefan: Similar to a triangle list but instead the first vertex is part of all the triangles. Also, similar to a triangle strip, each vertex creates a new triangle, connected to the previous vertex.