Small test-app playing ground for text rendering

The test app currently shows a number of problems in text rendering

* Above a certain font-size, there is no glyph caching, which
also means there is no sub-pixel filtering. It is actually
the same problem right now for all vector-drawing. It uses the
sub-pixel rendering pipeline, but without any filtering.
* This test feels very sluggish. Something is going on which seems
to be very unoptimized. To be observed when resizing the window
and causing redraws.
* Overlapping glyphs are handled wrongly in B_OP_COPY mode. The
problem is that glyphs are drawn one at a time, but B_OP_COPY
also draws the background color. In case some pixels already
drawn with the text color by one glyph are drawn again by the
next glyph, but with the background color, then it looks like
gaps in the text. This problem has always existed. With the
double filtering that was recently fixed, it was just especially
visible, since, due to the smearing, it was much more likely that
there were extra background pixels at the left side of glyphs.
(Right side, too, but without visual effect.) Of course not
completely transparent pixels, those are not drawn, but almost
transparent pixels.

Change-Id: If590a4c93983c5a3ff0772c19074c408be8e1e74
Reviewed-on: https://review.haiku-os.org/c/871
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>