Add support for arguments to do_in_context

So that std::bind isn't needed.

Closes #33.
Add texture coordinates and normals for rectangles
Remove the useless "size" member of rectangle
Have rectangle take floats instead of integers
Add texture class
Use the texture class in cameras
Make textures use the new activatable class
Fix setting texture parameters without a context

Closes #32.
Fix setting texture parameters without an active context

Closes #35.
Use do_in_context parameters in texture class
Use renamed activatable methods in texture class