Change blackfill color to transparant black for scalable picons

When picons do have a (8 bit) color map without any transparant colors we
had a white blackfill when the picons are scaled and centered into the
destination. Hopefully backfilling in transparant black will be a better
work-a-round. However for correct displaying it still should be considered
to use non (8-bit) color mapped picons.
gdi/gpixmap: Don't use broken hardware alphablending

Alphablending on (at least) the ET boxes is broken. When one of the
pixels is fully transparent, the resulting pixel is always fully
transparent instead of simply the other pixel's argb values.

This resulted in objects not being rendered at all, when the background
is transparent. It is also the reason why a transparent picon will occur
as a "hole in the UI" instead of just being invisible.

As a workaround, don't use hardware alphablending (even if the hardware
reports it as a capability) except when scaling. Historically, this has
been the only use for hardware accellerated blending anyway, so there's
no loss in performance.
gdi/picexif: Remove unused variable "data_precision"
gpixmap: Copy palette to hardware only once.

Keep track of physical RGB table, and convert it only once. This
assumes that no one adapts the palette after having rendered it.

This prevents having to process up 1k of data for each hardware
blit.
Merge remote-tracking branch 'pli/master'