vcl: Allow SalGraphics to draw gradients natively

The aim of this patch is to allow for native gradient rendering in
SalGraphics (i.e. let OpenGL do this natively). It is a two step
process:

1. I need to allow gradient draw into SalGraphics, however the current
completely intertwined with the metafile code in OutputDevice. I am
seperating the gradient metafile code from the gradient drawing code.

2. After splitting the metafile stuff from the actual gradient drawing,
I am now able to call on SalGraphics::DrawGradient(). This just
calls on SalGraphics::drawGradient() which returns false if there is
no way of drawing native gradients, and true if there is. If false,
then we use OutputDevice's DrawGradient() functionality.

Change-Id: Ibaaabe13b76a8e7a037d9f751b5f662653a50566
Reviewed-on: https://gerrit.libreoffice.org/12119
Reviewed-by: Chris Sherlock <chris.sherlock79@gmail.com>
Tested-by: Chris Sherlock <chris.sherlock79@gmail.com>