Make rectangle rendering pixel-perfect

Offsetting two of the borders renders all of the corners (previously,
the top-left pixel was left out) and ensures that semitransparent pixels
are not double-rendered.

Pixel-perfection still depends on drivers not stuffing up line
rendering; 0.5px should probably be added to all coordinates at some
point.

Author: aidanholm

crawl-ref/source/tilebuf.cc

@@ -318,8 +318,8 @@ void LineBuffer::add(float pos_sx, float pos_sy, float pos_ex, float pos_ey,
 void LineBuffer::add_square(float sx, float sy, float ex, float ey,
                             const VColour &col)
 {
-    add(sx, sy, ex, sy, col);
-    add(ex, sy, ex, ey, col);
+    add(sx-1, sy, ex-1, sy, col);
+    add(ex, sy-1, ex, ey-1, col);
     add(ex, ey, sx, ey, col);
     add(sx, ey, sx, sy, col);
 }