Double buffering default is false, then can be enabled on a per-widget
basis, finally this can be overriden at runtime by the
VCL_DOUBLEBUFFERING_FORCE_ENABLE environment variable (so that
everything is painted using double buffering).

Let SupportsDoubleBuffering() also respect this variable, so code
calling SupportsDoubleBuffering() can react to the runtime override,
too.

Change-Id: Ic9a1c00a801f6976069d7cfc47c3fa491ebc1ff0