Add Curve dialog channel images to resource file

In preparation for testing button strip image support
loadResourceToDIB: auto-instantiate source DIB as necessary
New function for quickly scaling RGB values by some constant
pdDIB: fix premultiplication issue when compositing a background color
Button strip UC: add button image support

Disabled and hover state are automatically handled by the control, which
is great!
Curves tool: switch channel selector to button strip

Button images need to be revised, but they were sufficient for testing
purposes.
Button strip UC: adjust bounds-checking behavior of arrow keys