Use QSGGeometry::lineWidth() when drawing GL_POINTS

Since glLineWidth doesn't affect the drawing of GL_POINTS, it makes sense
to use the lineWidth member to affect point size when drawing points.

[ChangeLog][QtQuick][QSGGeometry] Changed QSGGeometry::lineWidth
to also affect point size (glPointSize) when drawing GL_POINTS, in
addition to the existing behavior of affecting line width when drawing
GL_LINES, GL_LINE_STRIP, and GL_LINE_LOOP.

Task-number: QTBUG-35772
Change-Id: I0ea73f9261509e8d356f91a063dc90dc0f8980bd
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>