Improve check for whether canvas is visible

As seen with https://stackoverflow.com/questions/57676500 scaling the
canvas causes problems for glslCanvas.  This is because checking for
whether the canvas is visible mixes values from the bounding client rect
(which is affected by scaling), with the canvas element (which is not).

Use the bounding client rect to determine the visibility of the canvas.

dist/GlslCanvas.es.js

@@ -483,7 +483,8 @@ function parseUniforms(uniforms) {
 }
 
 function isCanvasVisible(canvas) {
-    return canvas.getBoundingClientRect().top + canvas.height > 0 && canvas.getBoundingClientRect().top < (window.innerHeight || document.documentElement.clientHeight);
+    var bound = canvas.getBoundingClientRect();
+    return bound.top + bound.height > 0 && bound.top < (window.innerHeight || document.documentElement.clientHeight);
 }
 
 function isPowerOf2(value) {

dist/GlslCanvas.js

@@ -888,7 +888,8 @@ function parseUniforms(uniforms) {
 }
 
 function isCanvasVisible(canvas) {
-    return canvas.getBoundingClientRect().top + canvas.height > 0 && canvas.getBoundingClientRect().top < (window.innerHeight || document.documentElement.clientHeight);
+    var bound = canvas.getBoundingClientRect();
+    return bound.top + bound.height > 0 && bound.top < (window.innerHeight || document.documentElement.clientHeight);
 }
 
 function isPowerOf2(value) {

lib/GlslCanvas.js

@@ -884,7 +884,8 @@ function parseUniforms(uniforms) {
 }
 
 function isCanvasVisible(canvas) {
-    return canvas.getBoundingClientRect().top + canvas.height > 0 && canvas.getBoundingClientRect().top < (window.innerHeight || document.documentElement.clientHeight);
+    var bound = canvas.getBoundingClientRect();
+    return bound.top + bound.height > 0 && bound.top < (window.innerHeight || document.documentElement.clientHeight);
 }
 
 function isPowerOf2(value) {

src/tools/common.js

@@ -1,6 +1,7 @@
 export function isCanvasVisible(canvas) {
-    return	((canvas.getBoundingClientRect().top + canvas.height) > 0) &&
-        (canvas.getBoundingClientRect().top < (window.innerHeight || document.documentElement.clientHeight));
+    let bound = canvas.getBoundingClientRect();
+    return	((bound.top + bound.height) > 0) &&
+        (bound.top < (window.innerHeight || document.documentElement.clientHeight));
 }
 
 export function isPowerOf2(value) {