Replace pixmap color inversion logic

We currently use our own code for this - which is erroneous and also is very slow (written in Python).
This fix falls back to using MuPDF native functions fz_invert_pixmap and fz_invert_pixmap_rect.
This obsoletes function "JM_invert_pixmap_rect" and significantly improves speed.

src/__init__.py

@@ -10185,8 +10185,10 @@ def invert_irect(self, bbox=None):
             return False
         r = JM_irect_from_py(bbox)
         if mupdf.fz_is_infinite_irect(r):
-            r = mupdf.fz_pixmap_bbox( pm)
-        return bool(JM_invert_pixmap_rect( pm, r))
+            mupdf.fz_invert_pixmap(pm)
+            return True
+        mupdf.fz_invert_pixmap_rect(pm, r)
+        return True
 
     @property
     def irect(self):
@@ -16282,42 +16284,6 @@ def JM_insert_font(pdf, bfname, fontfile, fontbuffer, set_simple, idx, wmode, se
             ]
     return value
 
-
-def JM_invert_pixmap_rect( dest, b):
-    '''
-    invert a rectangle - also supports non-alpha pixmaps
-    '''
-    assert isinstance( dest, mupdf.FzPixmap)
-    assert isinstance( b, mupdf.FzIrect)
-    b = mupdf.fz_intersect_irect(b, mupdf.fz_pixmap_bbox( dest))
-    w = b.x1 - b.x0
-    y = b.y1 - b.y0
-    if w <= 0 or y <= 0:
-        return 0
-
-    destspan = dest.stride()
-    destp = destspan * (b.y0 - dest.y()) + dest.n() * (b.x0 - dest.x())
-    n0 = dest.n() - dest.alpha()
-    alpha = dest.alpha()
-    while 1:
-        s = destp
-        for x in range( w):
-            for i in range( n0):
-                ss = mupdf.fz_samples_get( dest, s)
-                ss = 255 - ss
-                mupdf.fz_samples_set( dest, s, ss)
-                s += 1
-            if alpha:
-                ss = mupdf.fz_samples_get( dest, s)
-                ss += 1
-                mupdf.fz_samples_set( dest, s, ss)
-        destp += destspan
-        y -= 1
-        if y == 0:
-            break
-    return 1
-
-
 def JM_irect_from_py(r):
     '''
     PySequence to mupdf.FzIrect. Default: infinite irect