Agg: Increase maximum size to 2**23

With 32-bit scan lines, we are able to take advantage of the full 32-bit
range of coordinates. However, as noted in
`extern/agg24-svn/include/agg_rasterizer_scanline_aa.h`, Agg uses
24.8-bit fixed point coordinates. With one bit taken for the sign, the
maximum coordinate is now 2**23.

Author: QuLogic

doc/users/next_whats_new/increased_figure_limits.rst

@@ -0,0 +1,9 @@
+Increased Figure limits with Agg renderer
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Figures using the Agg renderer are now limited to 2**23 pixels in each
+direction, instead of 2**16. Additionally, bugs that caused artists to not
+render past 2**15 pixels horizontally have been fixed.
+
+Note that if you are using a GUI backend, it may have its own smaller limits
+(which may themselves depend on screen size.)

src/_backend_agg.cpp

@@ -33,10 +33,10 @@ RendererAgg::RendererAgg(unsigned int width, unsigned int height, double dpi)
         throw std::range_error("dpi must be positive");
     }
 
-    if (width >= 1 << 16 || height >= 1 << 16) {
+    if (width >= 1 << 23 || height >= 1 << 23) {
         throw std::range_error(
             "Image size of " + std::to_string(width) + "x" + std::to_string(height) +
-            " pixels is too large. It must be less than 2^16 in each direction.");
+            " pixels is too large. It must be less than 2^23 in each direction.");
     }
 
     unsigned stride(width * 4);