Add option to disable scaling altogether when height is less than hard limit

Cherry-picked from: jellyfin@bcf4b5f

Author: JustAMan

src/subtitles-octopus.js

@@ -20,7 +20,7 @@ var SubtitlesOctopus = function (options) {
     self.libassMemoryLimit = options.libassMemoryLimit || 0; // set libass bitmap cache memory limit in MiB (approximate)
     self.libassGlyphLimit = options.libassGlyphLimit || 0; // set libass glyph cache memory limit in MiB (approximate)
     self.targetFps = options.targetFps || 30;
-    self.prescaleTradeoff = options.prescaleTradeoff || 1.0; // render subtitles less than viewport when less than 1.0 to improve speed, render to more than 1.0 to improve quality
+    self.prescaleTradeoff = options.prescaleTradeoff || null; // render subtitles less than viewport when less than 1.0 to improve speed, render to more than 1.0 to improve quality; set to null to disable scaling
     self.softHeightLimit = options.softHeightLimit || 1080; // don't apply prescaleTradeoff < 1 when viewport height is less that this limit
     self.hardHeightLimit = options.hardHeightLimit || 2160; // don't ever go above this limit
 
@@ -674,7 +674,12 @@ var SubtitlesOctopus = function (options) {
     };
 
     function _computeCanvasSize(width, height) {
-        if (self.prescaleTradeoff > 1) {
+        if (self.prescaleTradeoff === null) {
+            if (height > self.hardHeightLimit) {
+                width = width * self.hardHeightLimit / height;
+                height = self.hardHeightLimit;
+            }
+        } else if (self.prescaleTradeoff > 1) {
             if (height * self.prescaleTradeoff <= self.softHeightLimit) {
                 width *= self.prescaleTradeoff;
                 height *= self.prescaleTradeoff;