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_local.h

@@ -2016,7 +2016,7 @@ void RB_EndSurface(void);
 void RB_CheckOverflow( int verts, int indexes );
 #define RB_CHECKOVERFLOW(v,i) if (tess.numVertexes + (v) >= SHADER_MAX_VERTEXES || tess.numIndexes + (i) >= SHADER_MAX_INDEXES ) {RB_CheckOverflow(v,i);}
 
-void R_DrawElements( int numIndexes, glIndex_t firstIndex );
+void R_DrawElements( int numIndexes, int firstIndex );
 void RB_StageIteratorGeneric( void );
 void RB_StageIteratorSky( void );
 void RB_StageIteratorVertexLitTexture( void );

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)));
 }

code/renderergl2/tr_vbo.c

@@ -687,8 +687,13 @@ vcq;
 // srfVert_t is 60 bytes
 // assuming each vert is referenced 4 times, need 16 bytes (4 glIndex_t) per vert
 // -> need about 4/15ths the space for indexes as vertexes
+#if GL_INDEX_TYPE == GL_UNSIGNED_SHORT
+#define VAOCACHE_VERTEX_BUFFER_SIZE (sizeof(srfVert_t) * USHRT_MAX)
+#define VAOCACHE_INDEX_BUFFER_SIZE (sizeof(glIndex_t) * USHRT_MAX * 4)
+#else // GL_UNSIGNED_INT
 #define VAOCACHE_VERTEX_BUFFER_SIZE (16 * 1024 * 1024)
 #define VAOCACHE_INDEX_BUFFER_SIZE (5 * 1024 * 1024)
+#endif
 
 typedef struct buffered_s
 {