document some env vars in VCL

Change-Id: I780e18b7afee470cff525904687746e163d30bdd

vcl/README.vars

+Environment variables in VCL:
+
+General
+-------
+SAL_USE_VCLPLUGIN - use a VCL plugin
+SAL_NO_NWF - disable native widgets
+SAL_FORCEDPI - force a specific DPI (gtk & gtk3 plugins only)
+
+VCL_DOUBLEBUFFERING_AVOID_PAINT - don't paint the buffer, useful to see where we do direct painting
+VCL_DOUBLEBUFFERING_FORCE_ENABLE - enable double buffered painting
+
+VCL_HIDE_WINDOWS - don't draw windows
+
+Bitmap
+------
+VCL_NO_THREAD_SCALE - disable threaded bitmap scale
+EMF_PLUS_DISABLE - use EMF rendering and ignore EMF+ specifics
+
+OpenGL
+------
+SAL_FORCEGL - force enable OpenGL
+SAL_WITHOUT_WIDGET_CACHE - disable LRU caching of native widget texutres