futurepress#942 Parsed the padding before adding as I found some book…

…s where these values are coming in as strings appended with px like "38px"

src/rendition.js

@@ -939,7 +939,7 @@ class Rendition {
 
 		let computed = contents.window.getComputedStyle(contents.content, null);
 		let height = (contents.content.offsetHeight - (parseFloat(computed.paddingTop) + parseFloat(computed.paddingBottom))) * .95;
-		let horizontalPadding = computed.paddingLeft + computed.paddingRight;
+		let horizontalPadding = parseFloat(computed.paddingLeft) + parseFloat(computed.paddingRight);
 
 		contents.addStylesheetRules({
 			"img" : {