Add support for margins in Webtoon view

On larger tablets, matching the page width to the screen width in
webtoon mode causes eye strain due to the image looking so magnified.
Adding a page margin to the image can resolve this by effectively
scaling the image down.

app/src/main/java/eu/kanade/tachiyomi/data/preference/PreferenceKeys.kt

@@ -51,6 +51,8 @@ object PreferenceKeys {
 
     const val readWithVolumeKeysInverted = "reader_volume_keys_inverted"
 
+    const val webtoonMarginRatio = "margin_ratio"
+
     const val portraitColumns = "pref_library_columns_portrait_key"
 
     const val landscapeColumns = "pref_library_columns_landscape_key"

app/src/main/java/eu/kanade/tachiyomi/data/preference/PreferencesHelper.kt

@@ -79,6 +79,8 @@ class PreferencesHelper(val context: Context) {
 
     fun readWithVolumeKeysInverted() = rxPrefs.getBoolean(Keys.readWithVolumeKeysInverted, false)
 
+    fun marginRatio() = rxPrefs.getInteger(Keys.webtoonMarginRatio, 0)
+
     fun portraitColumns() = rxPrefs.getInteger(Keys.portraitColumns, 0)
 
     fun landscapeColumns() = rxPrefs.getInteger(Keys.landscapeColumns, 0)