Visible at e.g. in Calc: Format Cells/Borders/Line Styles.

The problem was that first commit
2e99e4e1 (opengl: use MVP matrix in
vertex shaders, pixel offsets, 2015-07-08) introduced the concept of
pixel offsets, setting the value (implicitly) to 0 in
OpenGLSalGraphicsImpl::DrawTrapezoid(), but using 0.5 in
OpenGLSalGraphicsImpl::FlushLinesOrTriangles().

This is fine, but then later commit
2003076c (opengl: batch draw
polypolygons, 2016-05-29) changed
OpenGLSalGraphicsImpl::drawPolyPolygon() to use deferred drawing instead
of DrawTrapezoid(), without doing any translation of the input
polypolygon. This resulted in loss of those polygons when used in the
above mentioned dialog, which has a listbox of bitmaps, where each line
style preview is drawn on a virtual device with a height of 1px. So at
the end the 1px offset meant the previews were simply missing.

('make CppunitTest_vcl_gen SAL_USE_VCLPLUGIN=gen SAL_FORCEGL=1' is
needed on Linux to see the test failing without the fix.)

Change-Id: Ia9f3d6e7cb38a43fe2f8a41746b538af68add43c
Reviewed-on: https://gerrit.libreoffice.org/69920Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins