fix(ui_compositor): only reset skipstart at first column (#24776)

Problem:    A double-width char in a floating window causes an extra
            space to be drawn to the left of its boundary.
Solution:   Only reset skipstart at the first column.

Fix #24775

src/nvim/ui_compositor.c

@@ -402,7 +402,7 @@ static void compose_line(Integer row, Integer startcol, Integer endcol, LineFlag
       if (col == endcol - 1) {
         skipend = 0;
       }
-    } else if (n > 1 && linebuf[col - startcol + 1][0] == NUL) {
+    } else if (col == startcol && n > 1 && linebuf[1][0] == NUL) {
       skipstart = 0;
     }