js: Simplify drawing context management

There's not much point in re-requesting the drawing context from the
offscreen canvas at the start of each drawing operation.  The canvas
keeps the context around and returns it on every call to getContext(),
so we may as well just keep our reference to it too.  This does mean
that the front-end won't detect puzzles drawing outside of a redraw
operation, but I think it's the mid-end's job to assert things like
that.

Rumours that I'm doing this because I had a mysterious bug whereby ctx
was unexpectedly null are entirely true.

2 files changed, 2 insertions, 3 deletions

diff --git a/emcclib.js b/emcclib.js
index 1be8979..3824293 100644
--- a/emcclib.js
+++ b/emcclib.js
@@ -224,7 +224,6 @@ mergeInto(LibraryManager.library, {
      * Prepare to do some drawing on the canvas.
      */
     js_canvas_start_draw: function() {
-        ctx = offscreen_canvas.getContext('2d', { alpha: false });
         update_xmin = update_xmax = update_ymin = update_ymax = undefined;
     },
 
@@ -267,7 +266,6 @@ mergeInto(LibraryManager.library, {
                                    update_xmax - update_xmin,
                                    update_ymax - update_ymin);
         }
-        ctx = null;
     },
 
     /*
diff --git a/emccpre.js b/emccpre.js
index bba40bc..6fba9b2 100644
--- a/emccpre.js
+++ b/emccpre.js
@@ -22,7 +22,7 @@
 var onscreen_canvas, offscreen_canvas;
 
 // A persistent drawing context for the offscreen canvas, to save
-// constructing one per individual graphics operation.
+// requesting it for each individual graphics operation.
 var ctx;
 
 // Bounding rectangle for the copy to the onscreen canvas that will be
@@ -245,6 +245,7 @@ function initPuzzle() {
     // Construct the off-screen canvas used for double buffering.
     onscreen_canvas = document.getElementById("puzzlecanvas");
     offscreen_canvas = document.createElement("canvas");
+    ctx = offscreen_canvas.getContext('2d', { alpha: false });
 
     // Stop right-clicks on the puzzle from popping up a context menu.
     // We need those right-clicks!