About memory consumption.
Now the UI demo eats ~2.4mb RAM. This is nearly ok.
BUT that doesn't include memory allocated by X process for pixmaps, window backing stores and other resources!
According to my measurements, X gets fatter by ~4.2mb. So the total is 6.5mb, that is not so ok...

UPD: I have measured memory size of minimal GTK program: 3.9 mb RSS + 0.6 mb X = 4.5 mb total. Yeah, it seems that i use too much pixmaps...
(but keep in mind that lxtest UI totally combined from pixmaps while gtk app needs no any)

auouymous, I have implemented your advise about scrolling. This really makes rotated scroll to be as fast as unrotated, thanks But there is an issue: If I blit already rotated window's content while scrolling, this breaks double-buffering... Copied window's content overrides overlapping transparent glow and then glow is redrawn on the top. So the glowing around seekbar's thumb starts flickering and this looks very ugly.
Of course, this can be easily solved by adding one more buffer in the current chain that will store rotated content. But one more 800x480x32bpp buffer will add ~1.5 mb to the X memory...