LibGfx/JPEG2000/IDWT: Call PSE only when needed in _1D_EXTR

Takes `Build/lagom/bin/image -o out.tiff ~/Downloads/balloon.jpf` from
586.0 ms ± 5.6 ms to 547.7 ms ± 5.1 ms under hyperfine on my system
(6.6% less). When run under `/usr/bin/time -l`, the changes are:

Before:

         14557698316  instructions retired
          2500654210  cycles elapsed

After:

         13958498350  instructions retired
          2318667968  cycles elapsed

The lossless files goes from 1.025 s to 1.000 s (likely smaller win
because it's half the dimensions).

(Note that the "from" hyperfine number is pretty different from the
previous commit, probably due to my system being under different load,
at different battery level, or what. But the "instructions retired"
are very comparable, so that's probably a better number to look at.)

Userland/Libraries/LibGfx/ImageFormats/JPEG2000InverseDiscreteWaveletTransform.h

@@ -275,7 +275,13 @@ inline void _1D_EXTR(Transformation transformation, IDWTOutput& a, int start, in
         return i0 + min(mod(i - i0, 2 * (i1 - i0 - 1)), 2 * (i1 - i0 - 1) - mod(i - i0, 2 * (i1 - i0 - 1)));
     };
 
-    for (int l = i0 - i_left, i = 0; l < i1 + i_right; ++l, ++i)
+    // PSE is the identity function for i0 <= i < i1, so only call it on the edges.
+    int i = 0;
+    for (int l = i0 - i_left; l < i0; ++l, ++i)
+        buffers.scanline_buffer[buffers.scanline_runway + i] = a.data[start + (PSE(l, i0, i1) - i0) * delta];
+    for (int l = i0; l < i1; ++l, ++i)
+        buffers.scanline_buffer[buffers.scanline_runway + i] = a.data[start + (l - i0) * delta];
+    for (int l = i1; l < i1 + i_right; ++l, ++i)
         buffers.scanline_buffer[buffers.scanline_runway + i] = a.data[start + (PSE(l, i0, i1) - i0) * delta];
 
     buffers.scanline_start = buffers.scanline_runway + i_left;