Fixed the MPRVolumeStyle, the node is not a composable style
Merge pull request #366 from aarbelaiz/volren-mpr-fix

Fixed the MPRVolumeStyle, the node is not a composable style.
- fix picking texture coordinates bug
- multipart fixes
- reset min/max after volume invalidate
Merge branch 'master' of https://github.com/x3dom/x3dom
Support for different attribute types in the texcoord- and color-picking shader
Changes to allow the multiple declarations of the same style in a volume data