Lazy texture uploading

This approach will create and upload the texture data only when the texture is really used (bind()) by the graphics pipeline. All the previous method of creation are still working as usual.

I've changed the text provider to use this new feature, and now the label texture is internally allocated during layouts operations = faster.

Is anybody could test this branch too? (anybody that doesn't have a nvidia card and/or with something else that the kivy demo)