Fix billboard example

(cherry picked from commit b143ceacd1405f897d00ab838a84d33a7f3e3784)

examples/Billboard/main.lua

@@ -1,5 +1,5 @@
 function lovr.load()
-  eye = lovr.graphics.newTexture('eye.png')
+  eye = lovr.graphics.newMaterial( lovr.graphics.newTexture('eye.png') )
 end
 
 function lovr.draw()