Agg: Use 32-bit scan line classes

When rendering objects, Agg rasterizes them into scan line objects (an
x/y point, horizontal length, and colour), and the renderer class writes
those to the pixels in the final buffer. Though we have determined that
Agg buffers cannot be larger than 2**16, the scan line classes that we
use contain 16-bit _signed_ integers internally, cutting off positive
values at half the maximum.

Since the renderer uses 32-bit integers, this can cause odd behaviour
where any horizontal span that _starts_ before 2**15 (such as a
horizontal spine) is rendered correctly even if it cross that point,
but those that start after (such as a vertical spine or any portion of
an angled line) end up clipped. For example, see how the spines and
lines break in matplotlib#28893.

A similar problem occurs for resampled images, which also uses Agg
scanlines internally. See the breakage in matplotlib#26368 for an example. The
example in that issue also contains horizontal spines that are wider
than 2**15, which also exhibit strange behaviour.

By moving to 32-bit scan lines, positions and lengths of the lines will
no longer be clipped, and this fixes rendering on very large figures.

Fixes matplotlib#23826
Fixes matplotlib#26368
Fixes matplotlib#28893

Author: QuLogic

src/_backend_agg.h

@@ -117,10 +117,10 @@ class RendererAgg
     typedef agg::renderer_scanline_bin_solid<renderer_base> renderer_bin;
     typedef agg::rasterizer_scanline_aa<agg::rasterizer_sl_clip_dbl> rasterizer;
 
-    typedef agg::scanline_p8 scanline_p8;
-    typedef agg::scanline_bin scanline_bin;
+    typedef agg::scanline32_p8 scanline_p8;
+    typedef agg::scanline32_bin scanline_bin;
     typedef agg::amask_no_clip_gray8 alpha_mask_type;
-    typedef agg::scanline_u8_am<alpha_mask_type> scanline_am;
+    typedef agg::scanline32_u8_am<alpha_mask_type> scanline_am;
 
     typedef agg::renderer_base<agg::pixfmt_gray8> renderer_base_alpha_mask_type;
     typedef agg::renderer_scanline_aa_solid<renderer_base_alpha_mask_type> renderer_alpha_mask_type;

src/_image_resample.h

@@ -712,6 +712,7 @@ void resample(
 
     using renderer_t = agg::renderer_base<output_pixfmt_t>;
     using rasterizer_t = agg::rasterizer_scanline_aa<agg::rasterizer_sl_clip_dbl>;
+    using scanline_t = agg::scanline32_u8;
 
     using reflect_t = agg::wrap_mode_reflect;
     using image_accessor_t = agg::image_accessor_wrap<input_pixfmt_t, reflect_t, reflect_t>;
@@ -739,7 +740,7 @@ void resample(
 
     span_alloc_t span_alloc;
     rasterizer_t rasterizer;
-    agg::scanline_u8 scanline;
+    scanline_t scanline;
 
     span_conv_alpha_t conv_alpha(params.alpha);