patch TinyRenderer so it software-renders in an OpenGL texture, for testing

fix texture support: flip texture to make OpenGL happy (lower-left is origin)
add path prefix to .obj loader, so materials/textures are loaded ok.

data/floor.obj

@@ -0,0 +1,18 @@
+o
+mtllib floor.mtl
+v -1 -1 -1
+v  1 -1 -1
+v  1 -1  1
+v -1 -1  1
+
+vt 0 0
+vt .25 0
+vt .25 .25
+vt 0 .25
+
+vn 0 1 0
+usemtl floor
+
+f 3/3/1 2/2/1 1/1/1
+f 4/4/1 3/3/1 1/1/1
+