Try to pick tRNS value that's valid at low depth (#520)

src/reduction/alpha.rs

@@ -57,8 +57,16 @@ pub fn reduced_alpha_channel(png: &PngImage, optimize_alpha: bool) -> Option<Png
     }
 
     let transparency_pixel = if has_transparency {
+        // For grayscale, start by checking 4 specific values in the hope that we may reduce depth
+        let unused = match png.ihdr.color_type {
+            ColorType::GrayscaleAlpha => [0x00, 0xFF, 0x55, 0xAA]
+                .into_iter()
+                .find(|&v| !used_colors[v as usize]),
+            _ => None,
+        }
+        .or_else(|| used_colors.iter().position(|&u| !u).map(|v| v as u8));
         // If no unused color was found we will have to fail here
-        Some(used_colors.iter().position(|b| !*b)? as u8)
+        Some(unused?)
     } else {
         None
     };

tests/reduction.rs

@@ -916,6 +916,18 @@ fn grayscale_alpha_8_should_be_grayscale_trns_8() {
     );
 }
 
+#[test]
+fn grayscale_alpha_8_should_be_grayscale_trns_1() {
+    test_it_converts(
+        "tests/files/grayscale_alpha_8_should_be_grayscale_trns_1.png",
+        true,
+        GRAYSCALE_ALPHA,
+        BitDepth::Eight,
+        GRAYSCALE,
+        BitDepth::One,
+    );
+}
+
 #[test]
 fn small_files() {
     test_it_converts(