Part of #4516: Windows renders start workers only for the cores the process may use

The renderer sized its worker pool from available_parallelism, which on
Windows counts every processor in the machine, also when the process's
affinity mask allows fewer. A process allowed one core started 8 workers,
and the renderer's flush spun on that core while the workers got no time,
so renders crawled or stalled.

The desktop app and vectorcraft-cli now read the affinity mask at startup
(src/windows_cores.rs, shared) and pass the count to the renderer with
vectorcraft_render::set_usable_cores. With one usable core the renderer
renders on the calling thread.

apps/vectorcraft-cli/src/main.rs

@@ -49,6 +49,10 @@ mod prefs_dir;
 #[cfg(windows)]
 #[path = "../../vectorcraft/src/system_fonts.rs"]
 mod system_fonts;
+/// The desktop app's affinity-mask reader, shared: renders start workers only for usable cores.
+#[cfg(windows)]
+#[path = "../../vectorcraft/src/windows_cores.rs"]
+mod windows_cores;
 
 use serde_json::{Value, json};
 use vectorcraft_engine::file_access::{self, AutomationRoots};
@@ -161,6 +165,9 @@ fn help() -> Result<(), String> {
 }
 
 fn main() -> ExitCode {
+    // Before the first render: the cores the process may run on.
+    #[cfg(windows)]
+    windows_cores::install();
     // Before any font lookup: the fonts font services load (#579).
     #[cfg(windows)]
     system_fonts::install();