Generate rasterized hatching at correct DPI.

The hatch buffer was created at 72 pixels, with the assumption that that
is the "unit square" (1in*1in). However, with default DPI increased to
100, that buffer is actually too small, causing hatching to be denser
than it should be. On test images where the DPI was set really low, the
hatching was less dense than it should be.

Fixes matplotlib#4108.

Author: QuLogic

doc/users/dflt_style_changes.rst

@@ -625,6 +625,11 @@ The behavior of the PS and Agg backends was DPI dependent, thus::
 
 There is no API level control of the hatch color or linewidth.
 
+Hatching patterns are now rendered at a consistent density, regardless of DPI.
+Formerly, high DPI figures would be more dense than the default, and low DPI
+figures would be less dense.  This old behavior cannot be directly restored,
+but the density may be increased by repeating the hatch specifier.
+
 
 .. _default_changes_font: