Bug 801: Refine 'blend' usage and modes (API-doc and demo-code)

- RegionRenderer: Make 'blend' setup pluggable via new GLCallbacks
  - 'GLCallback's for enable/disable, passed via 'create' method.
    Add 'defaultBlendEnable' and 'defaultBlendDisable',
    replacing previos fixed calls.

- GLRegion.draw(..) added API-doc notes about:
  - Decorating call with RegionRenderer.enable(..)
  - glClearColor impact and blending

- VBORegion2P*: Remove fixed glClearColor(..) call