README.opengl: Some notes about env. vars. useful for OpenGL

Change-Id: I25e50bdead1ad1e39be3d527685230c84f08a292

vcl/opengl/README.opengl

+Run LO with OpenGL enabled
+--------------------------
+SAL_USE_VCLPLUGIN=gtk SAL_FORCEGL=1 ./soffice
+
+Environment variables used:
+
+SAL_USE_VCLPLUGIN - use the specified VCL plugin (GTK2 in this case - currently
+needed on Linux because the default GTK3 doesn't support OpenGL yet)
+
+SAL_FORCEGL - enable OpenGL even if the card is blacklisted.
+
+Other variables:
+
+LIBGL_ALWAYS_SOFTWARE=1 - on Linux+Mesa forces the software renderer to be used
+(this is useful as an alternative to spot driver specific bugs)
+
+SAL_LOG=+INFO.vcl.opengl - if "--enable-dbgutil" is used, this can show OpenGL
+various rendering messages.
+
+LD_PRELOAD=/usr/lib64/apitrace/wrappers/glxtrace.so - preload the wrapper for
+APItrace. The path is the default used in Fedora 21+.
+
+Run VCLDemo
+-----------
+
+SAL_USE_VCLPLUGIN=gtk SAL_FORCEGL=1 ./bin/run vcldemo