fix watermark opacity for transparent watermarks (#1877)

CHANGELOG.md

@@ -66,6 +66,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Fix adding a single image to a manual collection from search results
 - Fix loading manual collection view when collection contains images
 - Fix edge case where block playout history would get stuck and repeat an item
+- Fix adjusting watermark opacity when watermark already contains alpha channel (is already transparent)
 
 ### Changed
 - Remove some unnecessary API calls related to media server scanning and paging

ErsatzTV.FFmpeg.Tests/Filter/WatermarkOpacityFilterTests.cs

@@ -26,7 +26,7 @@ public class WatermarkOpacityFilterTests
                 75,
                 false));
 
-        filter.Filter.Should().Be("colorchannelmixer=aa=0.75");
+        filter.Filter.Should().Be("format=yuva420p|yuva444p|yuva422p|rgba|abgr|bgra|gbrap|ya8,colorchannelmixer=aa=0.75");
     }
 
     [Test]
@@ -44,6 +44,6 @@ public class WatermarkOpacityFilterTests
                 75,
                 false));
 
-        filter.Filter.Should().Be("colorchannelmixer=aa=0.75");
+        filter.Filter.Should().Be("format=yuva420p|yuva444p|yuva422p|rgba|abgr|bgra|gbrap|ya8,colorchannelmixer=aa=0.75");
     }
 }

ErsatzTV.FFmpeg/Filter/WatermarkOpacityFilter.cs

@@ -14,7 +14,7 @@ public class WatermarkOpacityFilter : BaseFilter
         get
         {
             double opacity = _desiredState.Opacity / 100.0;
-            return $"colorchannelmixer=aa={opacity.ToString("F2", NumberFormatInfo.InvariantInfo)}";
+            return $"format=yuva420p|yuva444p|yuva422p|rgba|abgr|bgra|gbrap|ya8,colorchannelmixer=aa={opacity.ToString("F2", NumberFormatInfo.InvariantInfo)}";
         }
     }