GL slider changes:
  - Pressing the LMB moves the knob to the pointer.
  - Eliminate small knob jump after the parent window is moved.
  - Use rounding when snapping knob to integer positions.
GL: Implement blit of UR_RAST_RGB into UR_RAST_RGBA.
GL: Add item-view widget.  gui_parseArgs() can evaluate path!.
Fix GCC "dereferencing type-punned pointer" warnings.
GL: item-view glScissor handles dragging of parent window.
GL: Implement item-view image command.  gui_parseArgs() can evaluate paren!.
GL: Add color statement to item-view layout.

This uses the color vertex attribute and a CLUT in the gui-style/texture.
A use_color variable is required in the GUI shader to support both draw-prog!
and item-view VBOs.
GL item-view: Can select items and scroll view with mouse wheel.

Add gui_styleContext() so widgets can bind blocks to style.