Fixing culling for magnifier use case

Fixing culling for magnifier use case

Should use the camera's canvas size, not the viewport.

Added canvas size to left/right/top/bottom clipping distance
in camera for perspective projection

Fixed bug that would always force using perspective projection
Fixed bug with modifying scene object directly in event thread.

Top and Bottom clipping plane values for perspective have been
made consistent with values for Ortho projection; however, this
currently forces top to be +ve and bottom to be -ve when set
using just size. Test cases are inconsistent in this regard; when
set via l/r/t/b/n/f API, top and bottom are -ve and +ve respectively.
  Have used fabsf when re-calculating bounding box dimensions to
allow for this inconsistency.

Change-Id: I95e80e4c09f17a3240cccf16df23f09e8d9ad9cf