target-bsnes: Do not set the window background colour.

The bsnes window is mostly covered by the viewport, which is supposed to be
black. When the window is resized, the hiro Viewport widget repaints itself,
according to whatever hiro backend is in use.

However, on some platforms that's not enough. Specifically on X11, when the user
resizes a window the window manager, the X server, and the application can all
have different ideas about the exact window size. The X11 protocol gives each
window a "default background colour", to be used by the X server to fill the
blanks after the window manager has resized the window, but before the
application has been able to repaint it. Because the bsnes window is *mostly*
black, bsnes sets its window background colour to black, so the flickering of
X-server-drawn and application-drawn content will be less obvious.

Unfortunately, it gets more complex. GTK+2 was designed around the X11 model,
and so it works basically the way bsnes expects. GTK+3 was designed around the
more modern compositing model, which doesn't need that "default background
colour" hack, and as a result when you set the window background colour on
GTK+3. that background colour is used for *everything*. Even the menu-bar, which
by default draws text in a very dark grey, so you can hardly read what it says.

To fix this problem, we stop setting the background of the entire window to
black, and instead only set the background of the widgets in the middle of the
window. Specifically, the viewport widget is always black anyway, but when no
game is loaded bsnes has a Canvas widget that displays the bsnes icon.
Previously, a Canvas widget would show a background colour *or* an icon, but now
that an icon can have a padding color, we can use that to make sure the icon
background is black, regardless of the canvas size.

Fixes #108.

bsnes/target-bsnes/presentation/presentation.cpp

@@ -213,8 +213,8 @@ auto Presentation::create() -> void {
 
   iconLayout.setAlignment(0.0).setCollapsible();
   image icon{Resource::Icon};
-  icon.alphaBlend(0x000000);
-  iconCanvas.setIcon(icon);
+  iconCanvas.setIcon(icon, {0, 0, 0});
+  iconCanvas.setAlignment({0.0, 0.0});
   iconCanvas.setDroppable();
   iconCanvas.onDrop([&](auto locations) { onDrop(locations); });
 
@@ -250,7 +250,6 @@ auto Presentation::create() -> void {
   });
 
   setTitle({"bsnes v", Emulator::Version});
-  setBackgroundColor({0, 0, 0});
   resizeWindow();
   setAlignment(Alignment::Center);
 

bsnes/target-bsnes/presentation/presentation.hpp

@@ -126,7 +126,7 @@ struct Presentation : Window {
       Viewport viewport{&viewportLayout, Size{~0, ~0}, 0};
       VerticalLayout iconLayout{&viewportLayout, Size{0, ~0}, 0};
         Canvas iconSpacer{&iconLayout, Size{144, ~0}, 0};
-        Canvas iconCanvas{&iconLayout, Size{128, 128}, 0};
+        Canvas iconCanvas{&iconLayout, Size{144, 128}, 0};
     HorizontalLayout statusLayout{&layout, Size{~0, StatusHeight}, 0};
       Label spacerIcon{&statusLayout, Size{8, ~0}, 0};
       Canvas statusIcon{&statusLayout, Size{16, ~0}, 0};