ui: Improve dialog backgrounds.

Primarily, they now actually meet their contents, by being on the front
faces of their blocks; previously there was 15/16ths of a block of gap.

I also redrew the theme image. The new design has much less empty space
between the border and the content, is more rectangular, and has a faint
checkerboard pattern in the background. This checkerboard is not
necessarily the direction I want the art style to go, but as a temporary
measure, it will help when we’re debugging UI layout and want to count
how many cubes a widget is occupying, and it also adds some distraction
from widgets’ blocks’ shadows against the background.

In the future, I hope the dialog background will be able to be not just
2D, and will incorporate more shades of color. (Perhaps we will have a
complementary theme color that isn’t also the logo color.)

all-is-cubes-ui/src/vui/widgets/theme.rs

@@ -192,8 +192,10 @@ impl WidgetBlocks {
                             R64, // 16 res × 4 tiles
                             txn.insert_anonymous(space_from_image(
                                 include_image!("theme/dialog-background.png"),
-                                GridRotation::IDENTITY,
-                                &default_srgb,
+                                GridRotation::RXYZ,
+                                // place image on the front face (of the R16 individual blocks!)
+                                // so it meets the back of the widgets in the dialog.
+                                &|color| default_srgb(color).translate([0, 0, 15]),
                             )?),
                         )
                         .build()