When SAL_USE_VCLPLUGIN=svp is used, LO is expected to render
output only to bitmaps, without real GUI windows. This adds an
enabler and a getter function to the Application class, so one
can query this information easy as (Enable|Is)BitmapRendering.

This can be used by all VCL plugins, which can't fall back to
the Cairo based SVP plugin, primary OSX and Win.

A working implementation should allow to run all test via SSH.
All window-requiring tests already have to set this requirement
using gb_CppunitTest_use_vcl_non_headless(_with_windows)? and
should be moved to a different make target, or we need some test
harness to handle this correctly, before VCL fails the test.

Change-Id: I4bd4c81122a6686b090fdd93256d4712ac5f05dd
Reviewed-on: https://gerrit.libreoffice.org/64051
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>