OpenGL2: Fix world VAO cache drawing when glIndex_t is unsigned short

OpenGL ES is only required to support unsigned short for element buffer
values.

R_DrawElements() firstIndex argument was glIndex_t which caused element
indexes to wrap around to 0 when glIndex_t is an unsigned short.
(glIndex_t is an index into the vertexes buffer, not element buffer.)
Change it to 'int' like tess.firstIndex which is passed to
R_DrawElements().

World VAO cache buffer size allowed storing more vertexes than unsigned
short glIndex_t could reference. This resulted in the vertex indexes in
the element buffer wrapping around to 0.

code/renderergl2/tr_shade.c

@@ -38,7 +38,7 @@ R_DrawElements
 ==================
 */
 
-void R_DrawElements( int numIndexes, glIndex_t firstIndex)
+void R_DrawElements( int numIndexes, int firstIndex )
 {
 	qglDrawElements(GL_TRIANGLES, numIndexes, GL_INDEX_TYPE, BUFFER_OFFSET(firstIndex * sizeof(glIndex_t)));
 }