draw_elements: Separate count from the (optional) element list. Allow…

…s buffered drawing.

Fixes #13

gl2.rs

@@ -531,12 +531,18 @@ pub fn draw_arrays(mode: GLenum, first: GLint, count: GLsizei) {
     }
 }
 
-pub fn draw_elements(mode: GLenum, element_type: GLenum, indices: &[u8]) {
+pub fn draw_elements(mode: GLenum, count: GLsizei, element_type: GLenum, indices: Option<&[u8]>) {
     unsafe {
         return ll::glDrawElements(mode,
-                                  indices.len() as GLsizei,
+                                  match indices {
+                                    Some(ref i) => cmp::min(count, i.len() as GLsizei),
+                                    None => count,
+                                  },
                                   element_type,
-                                  cast::transmute(&indices[0]));
+                                  match indices {
+                                    Some(ref i) => cast::transmute(&i[0]),
+                                    None => ptr::null(),
+                                  })
     }
 }