Improve framing tool usability

* Rename "Resize" to "Resize & Framing" from discoverability
* Add ColorPreview class for displaying a solid color
* Add border color preview

rtengine/ipresize.cc

@@ -1069,7 +1069,9 @@ Imagefloat* ImProcFunctions::drawFrame(Imagefloat* rgb, const FramingParams& par
 
     Imagefloat* framed = new Imagefloat(dims.framedWidth, dims.framedHeight);
 
-    auto clip = [](int v) { return std::max(0, std::min(v, 65535)); };
+    // Color::gamma2curve expects a 16-bit value, but the GUI sliders are
+    // using 8-bit values. Step up the user value to 16-bits.
+    auto clip = [](int v) { return std::max(0, std::min(v, 255)) * 256; };
 
     float r = Color::gamma2curve[clip(params.borderRed)];
     float g = Color::gamma2curve[clip(params.borderGreen)];