Use Element.textContent for StringContentHandler
Expose cameraPosition as a shader parameter using this.cameraPosition.

Signed-off-by: Jan Sutter <jan.sutter@dfki.de>
Fix bug in XML3D.debug.assert

Signed-off-by: Jan Sutter <jan.sutter@dfki.de>
Store source width and height in GLTexture.

Signed-off-by: Jan Sutter <jan.sutter@dfki.de>