js: Allow CSS to set the font used by the puzzle

This means that the calculated font properties of the HTML canvas now
control what font is used.  The size is overridden, and for monospaced
text so is the family.

I'd like to be able to also specify the monospaced font, maybe using a
CSS variable, but that looks like being quite a lot of extra complexity.

My experience when testing this was that constructing a valid "font"
string for a canvas context is prone to breakage, but broke in a way
that left the font unchanged, so we always set a simple specification
first before trying to construct one from CSS.

emccpre.js

@@ -179,6 +179,22 @@ function canvas_mouse_coords(event, element) {
     return {x: rcoords.x * xscale, y: rcoords.y * yscale}
 }
 
+// Set the font on a CanvasRenderingContext2d based on the CSS font
+// for the canvas, the requested size, and whether we want something
+// monospaced.
+function canvas_set_font(ctx, size, monospaced) {
+    var s = window.getComputedStyle(onscreen_canvas);
+    // First set something that we're certain will work.  Constructing
+    // the font string from the computed style is a bit fragile, so
+    // this acts as a fallback.
+    ctx.font = `${size}px ` + (monospaced ? "monospace" : "sans-serif");
+    // In CSS Fonts Module Level 4, "font-stretch" gets serialised as
+    // a percentage, which can't be used in
+    // CanvasRenderingContext2d.font, so we omit it.
+    ctx.font = `${s.fontStyle} ${s.fontWeight} ${size}px ` +
+        (monospaced ? "monospace" : s.fontFamily);
+}
+
 // Enable and disable items in the CSS menus.
 function disable_menu_item(item, disabledFlag) {
     item.disabled = disabledFlag;