vcl: copyBits should operate on mrParent if no context supplied.

eg. OutputDevice::DrawOutDev:
            mpGraphics->CopyBits( aPosAry, NULL, this, NULL );

Change-Id: I0b041bb5aa6aba573b9f589842084722481cb438
vcl: always use the default windows' GL Context for now to create bitmaps.

Change-Id: Ie20b10656788113709b0b0720d3cae2653639d78
vcldemo: improve my rather trivial inferred alpha maths.

Change-Id: I420628cea2272d6f5af0b13b9c32902627dcf86f