It's like DrawArrays, but the count is taken from a transform feedback
object.
This removes DrawTransformFeedback from dd_function_table and adds the same
function to GLvertexformat (with the function parameters matching GL).
The vbo_draw_func callback has a new parameter
"struct gl_transform_feedback_object *tfb_vertcount".
The rest of the code just validates states and forwards the transform
feedback object into vbo_draw_func.
14bb957b99