[poppler] Default Qt frontend image formats

[Adam Reichold]

Hello,

we recently changed the default image format produced by the Qt frontend
to RGB888 since that is improves rendering speed within Splash and
yields a smaller QImage (and hence less copying for the Qt4 frontend).

However, doing some heap profiling, I noticed that this change should
probably be reverted since this implies a conversion and hence at least
one additional copy when converting the resulting QImage to a QPixmap
with most Qt platforms, especially with Xcb using the Raster pixmap
implementation. The increase in maximum working set size this yields
will surely compensate any memory size savings of using RGB888 images as
a source.

Also, what we gain in speed during rendering, we would probably loose to
some degree even when painting these RGB888 images directly, since Qt's
Raster paint engine has no optimized blending functions for RGB888.

The previous default format of ARGB32 was also suboptimal in this
regards, since it would still imply that QRasterPlatformPixmap detaches
from the QImage's data yielding an additional copy on which it would
just change the format from ARGB32 to RGB32 after explicitly checking
that it was actually given opaque data.

Hence I think the correct choice is RGB32 for opaque pixel data since
this has optimized blending functions and QRasterPlatformPixmap will
neither convert nor copy this as it will usually coincide with the
screens native format (as per QNativeImage::systemFormat).

For IgnorePaperColor, the same can be achieved by choosing
ARGB32_Premultiplied instead of ARGB32 which will also not be copied or
converted by the Raster pixmap implementation.

Attached is a patch that makes those two changes yielding significant
reductions in maximum working set size. Also note that while the
premultiplication does have a performance impact, it slows down the
IgnorePaperColor path by less than one percent w.r.t. the default path.

Best regards, Adam.
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