Add tessellation data to DerivedMesh (LoopTri)

This stores loop indices into the loop array giving easier acess
to data such as vertex-colors and UV's,
removing the need to store an MFace duplicate of custom-data.

This doesn't yet move all internal code from MFace to LoopTri just yet.

Only applies to:
- opengl drawing
- sculpting (pbvh)
- vertex/weight paint

Thanks to @psy-fi for review, fixes and improvements to drawing!

source/blender/blenlib/intern/math_geom.c

@@ -3549,6 +3549,40 @@ void map_to_sphere(float *r_u, float *r_v, const float x, const float y, const f
 
 /********************************* Normals **********************************/
 
+void accumulate_vertex_normals_tri(
+        float n1[3], float n2[3], float n3[3],
+        const float f_no[3],
+        const float co1[3], const float co2[3], const float co3[3])
+{
+	float vdiffs[3][3];
+	const int nverts = 3;
+
+	/* compute normalized edge vectors */
+	sub_v3_v3v3(vdiffs[0], co2, co1);
+	sub_v3_v3v3(vdiffs[1], co3, co2);
+	sub_v3_v3v3(vdiffs[2], co1, co3);
+
+	normalize_v3(vdiffs[0]);
+	normalize_v3(vdiffs[1]);
+	normalize_v3(vdiffs[2]);
+
+	/* accumulate angle weighted face normal */
+	{
+		float *vn[] = {n1, n2, n3};
+		const float *prev_edge = vdiffs[nverts - 1];
+		int i;
+
+		for (i = 0; i < nverts; i++) {
+			const float *cur_edge = vdiffs[i];
+			const float fac = saacos(-dot_v3v3(cur_edge, prev_edge));
+
+			/* accumulate */
+			madd_v3_v3fl(vn[i], f_no, fac);
+			prev_edge = cur_edge;
+		}
+	}
+}
+
 void accumulate_vertex_normals(
         float n1[3], float n2[3], float n3[3], float n4[3],
         const float f_no[3],