Rename ol.structs.Buffer to ol.webgl.Buffer

The Buffer class is WebGL specific, and it's not really a "data structure", in the pure sense of the term.
Merge pull request #45 from elemoine/webgl-point-buffer

[webgl-point] Rename ol.structs.Buffer to ol.webgl.Buffer
Delete WebGL resources used at postcompose time
Delete WebGL resources used by immediate API