Refactored buffer.py according to the discussion with Nico
We had a chat yesterday to discuss the API of the buffer.
We decided on two attributes:
count: the number of vertices
vsize: the number of elements in the vector (like float, vec2, vec3, vec4)
Dtype, vsize and stride are fixed on initialization. Count can
be changed afterwards using set_count or set_data.
I like this approach. The code is now much cleaner and shorter.
NOTICE: some demos still fail, as well as some unit tests.
I will fix these next week. At least it should be better than previous
commit.