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.mtl

@@ -0,0 +1,13 @@
+newmtl floor
+  Ns 10.0000
+  Ni 1.5000
+  d 1.0000
+  Tr 0.0000
+  Tf 1.0000 1.0000 1.0000 
+  illum 2
+  Ka 0.0000 0.0000 0.0000
+  Kd 0.5880 0.5880 0.5880
+  Ks 0.0000 0.0000 0.0000
+  Ke 0.0000 0.0000 0.0000
+  map_Ka floor_diffuse.jpg
+  map_Kd cube.tga