Fix z-fighting on bold printout borders.

- Changed page background to render as one quad, instead of two halves.
- Set page background to a z-offset that is between zeroth (potentially
  bold border) and subsequent background pages. Bold borders were at the
  same z-offset before.

src/main/java/dan200/computercraft/client/render/PrintoutRenderer.java

@@ -115,8 +115,11 @@ public final class PrintoutRenderer
             }
         }
 
-        // Left half
-        drawTexture( transform, buffer, x, y, z, X_FOLD_SIZE * 2, 0, X_SIZE / 2.0f, Y_SIZE, light );
+        // Current page background: Z-offset is interleaved between the "zeroth" left/right page and the first
+        // left/right page, so that the "bold" border can be drawn over the edge where appropriate.
+        drawTexture( transform, buffer, x, y, z - 1e-3f * 0.5f, X_FOLD_SIZE * 2, 0, X_SIZE, Y_SIZE, light );
+
+        // Left pages
         for( int n = 0; n <= leftPages; n++ )
         {
             drawTexture( transform, buffer,
@@ -127,8 +130,7 @@ public final class PrintoutRenderer
             );
         }
 
-        // Right half
-        drawTexture( transform, buffer, x + X_SIZE / 2.0f, y, z, X_FOLD_SIZE * 2 + X_SIZE / 2.0f, 0, X_SIZE / 2.0f, Y_SIZE, light );
+        // Right pages
         for( int n = 0; n <= rightPages; n++ )
         {
             drawTexture( transform, buffer,