Update viewport code with new GDI+ profiling results

After a bunch of profiling on my Win 7 PC, I can state with great
confidence that the HighQualityBicubic interpolation mode is the fastest
mode for downsizing 32bpp images via GDI+.  I have no idea why this is,
but many, many iterative tests confirmed it.

Stranger still, in descending order, the fastest algorithms are as
follows.  (Testing done on a 6 megapixel, 5 layer image with tons of
varying alpha.)

NearestNeighbor (obviously) - < 10 ms per frame
HighQualityBicubic - mid 40's
HighQualityBilinear - mid 50's
Bilinear - low 60's
Bicubic - 150+ ms per frame

Regular bicubic interpolation is some 4x slower than the high quality
mode, which makes NO FRIGGIN' SENSE.  Also, how badly do you have to
write a bilinear interpolation algorithm to have it outperformed
significantly by a bicubic one??

Anyway, I'm happy to put this matter to rest.  It's possible that
different hardware/software combinations could cause a different result,
but I'm content to stick with HighQualityBicubic for now, especially
since it makes the viewport noticeably smoother while also improving
render quality while zoomed out.
Fix Transparency menu options to work with layers

All transparency options should now work just fine with Layers.

(Note also that all transparency options are now located in the Layers
menu instead of the Image menu.)