j3dcore: add members and helpers for future FBOlayer work

Extracted from a patch by August Lammersdorf.

[HSH - any mistakes here are mine]
Signed-off-by: August Lammersdorf <aldorf@interactivemesh.com>
Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
j3dcore: add new Pipeline method resizeOffscreenLayer

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>