Fix the visualization of unmerged batch geometries without index

indexData() is null for most QSGGeometryNodes subclasses in QtQuick,
make sure that we draw using vertex array data in that case.

The patch also increases the contrast of the line pattern, since it
is almost invisible with brighter hue values, to make it easier to
see which batches aren't merged.

Change-Id: Ife7b26692ee318feb0810ee0e787289eb151ea8a
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>