[util/gpu] Guard D3D11 atlas allocation (#6043)

The D3D11 demo atlas backend advanced its cursor and copied glyph data into the
shadow buffer without checking the fixed atlas capacity. Long sessions could
therefore write past the end of the shadow buffer once the atlas filled.

Match the other demo atlas backends by failing before advancing the cursor when
the requested glyph data does not fit.

Testing: meson test -C build
Assisted-by: OpenAI Codex <codex@openai.com>

util/gpu/demo-renderer-d3d11.cc

diff --git a/util/gpu/demo-renderer-d3d11.cc b/util/gpu/demo-renderer-d3d11.cc
index c29ec62..def5dd6 100644
--- a/util/gpu/demo-renderer-d3d11.cc
+++ b/util/gpu/demo-renderer-d3d11.cc
@@ -167,6 +167,8 @@
     backend.alloc = [](void *ctx_, const char *data, unsigned len_bytes) -> unsigned {
       auto *self = (demo_renderer_d3d11_t *) ctx_;
       unsigned len_texels = len_bytes / 8;
+      if (self->atlas.cursor + len_texels > self->atlas.capacity)
+	die ("Ran out of atlas memory");
       unsigned offset = self->atlas.cursor;
       self->atlas.cursor += len_texels;
       const int16_t *src = (const int16_t *) data;