New functions for padding, blurring, and drop-shadowing layers

All were helpful additions for adding drop shadows to the image
tabstrip.
Add slight drop shadows to the image tabstrip

Drop shadows are blurred in real-time using GDI+ if available, or a pure
PD fallback.  By my testing on an old XP machine, it still handles
taskbar resizing with ease, even when many images are loaded.

For 32bpp images, the drop shadow conforms to the actual image shape,
ignoring fully transparent regions!