Respect maxCanvasPixels when computing canvas dimensions

Ensure that we never round the canvas dimensions above `maxCanvasPixels`
by rounding them to the preceeding multiple of the display ratio rather
than the succeeding one.

web/ui_utils.js

@@ -260,6 +260,7 @@ function binarySearchFirstItem(items, condition, start = 0) {
  *  @param {number} x - Positive float number.
  *  @returns {Array} Estimated fraction: the first array item is a numerator,
  *                   the second one is a denominator.
+ *                   They are both natural numbers.
  */
 function approximateFraction(x) {
   // Fast paths for int numbers or their inversions.
@@ -306,9 +307,12 @@ function approximateFraction(x) {
   return result;
 }
 
-function roundToDivide(x, div) {
-  const r = x % div;
-  return r === 0 ? x : Math.round(x - r + div);
+/**
+ * @param {number} x - A positive number to round to a multiple of `div`.
+ * @param {number} div - A natural number.
+ */
+function floorToDivide(x, div) {
+  return x - (x % div);
 }
 
 /**
@@ -863,6 +867,7 @@ export {
   DEFAULT_SCALE_DELTA,
   DEFAULT_SCALE_VALUE,
   docStyle,
+  floorToDivide,
   getActiveOrFocusedElement,
   getPageSizeInches,
   getVisibleElements,
@@ -881,7 +886,6 @@ export {
   ProgressBar,
   removeNullCharacters,
   RenderingStates,
-  roundToDivide,
   SCROLLBAR_PADDING,
   scrollIntoView,
   ScrollMode,