Added a new global variable, DisabledDrawEffectStyle, that controls how TRawImage.PerformEffect(gdeDisabled) works to improve the appearance on colored backgrounds and in dark mode apps.

There are three possible values:
- ddesGrayscale
    This is the default and matches the existing behavior of just converting the pixels to gray.
- ddesDarken
    Convert the image to fully black pixels with alpha transparency.  This roughly matches ddesGrayscale on traditional gray backgrounds but lets colored backgrounds through.
- ddesLighten
    Convert the image to fully white pixels with alpha transparency.  This is the dark mode equivalent of ddesDarken.

Existing alpha transparency is factored into the conversion.  The constants used for the visibility calculation roughly match what other macOS apps use.

components/lazutils/graphtype.pp

@@ -268,6 +268,10 @@ function RawImageQueryFlagsToString(AFlags: TRawImageQueryFlags): string;
 
 var
   MissingBits: array[0..15] of array[0..7] of word;
+  // Control whether gdeDisabled is a grayscale image (classic), or uses
+  // alpha-transparent black or alpha-transparent white, improving appearance
+  // on non-grey light and dark backgrounds
+  DisabledDrawEffectStyle: (ddesGrayscale, ddesDarken, ddesLighten) = ddesGrayscale;
 
 implementation
 
@@ -1651,6 +1655,7 @@ var
   AData: PRGBAQuad;
   P: Pointer;
   i, j: integer;
+  Gray: Byte;
 begin
   // check here for Description. Only RGBA data can be processed here.
   if not CheckDescription then
@@ -1676,9 +1681,25 @@ begin
           begin
             with AData^ do
             begin
-              Red := (Red + Green + Blue) div 3;
-              Green := Red;
-              Blue := Red;
+              Gray := (Red + Green + Blue) div 3;
+              if DisabledDrawEffectStyle = ddesLighten then
+              begin
+                // Apply existing alpha and reduce visibility by a further 66%
+                Alpha := Byte((Integer(Gray) * Alpha) div 768);
+                Gray := $FF;
+              end
+              else begin
+                Gray := Byte(Integer(Gray) + Integer((DimPercent * (DimColor - Gray)) div 100));
+                if DisabledDrawEffectStyle = ddesDarken then
+                begin
+                  // Apply existing alpha and Reduce visibility by a further 25%
+                  Alpha := Byte((Integer($FF - Gray) * Alpha) div 384);
+                  Gray := $00;
+                end;
+              end;
+              Red := Gray;
+              Green := Gray;
+              Blue := Gray;
             end;
             inc(AData);
           end;