Fix vertex and texture array code in paint.cpp.

PrivateGLScreen::paintBackground(): If background textures are empty,
fix the vertex array pointer to use the beginning of the vertex data
rather than (data + 2). Fix data allocation so only the necessary number
of vertices is allocated. Disable the unspecified texture coordinate
array in this case, since leaving it enabled may cause the GL driver to
segfault when attempting to dereference it.