Make item icons transparent

tools/validate_data.ps1

@@ -877,7 +877,9 @@ function Check-Item-Effects() {
         $item = $itemProp.Value
         $kind = [string](Get-Prop $item "kind" "")
         if (Has-Prop $item "icon") {
-            Check-Portrait-Path (Get-Prop $item "icon" "") "Item $itemId icon"
+            $iconPath = [string](Get-Prop $item "icon" "")
+            Check-Portrait-Path $iconPath "Item $itemId icon"
+            Check-Item-Icon-Cutout $iconPath "Item $itemId icon"
         }
         if (-not $validItemKinds.Contains($kind)) {
             Fail "Item $itemId has unknown kind: $kind"
@@ -1874,6 +1876,51 @@ function Check-Portrait-Image([string]$AbsolutePath, [string]$Portrait, [string]
     }
 }
 
+function Check-Item-Icon-Cutout([string]$Icon, [string]$Context) {
+    $relativePath = $Icon.Substring("res://".Length)
+    $absolutePath = [System.IO.Path]::GetFullPath((Join-Path ([System.IO.Path]::GetFullPath($Root)) $relativePath))
+    $bitmap = $null
+    try {
+        $bitmap = [System.Drawing.Bitmap]::FromFile($absolutePath)
+        if (-not [System.Drawing.Image]::IsAlphaPixelFormat($bitmap.PixelFormat)) {
+            Fail "$Context must be an alpha PNG cutout: $Icon"
+        }
+        $cornerPixels = @(
+            $bitmap.GetPixel(0, 0),
+            $bitmap.GetPixel($bitmap.Width - 1, 0),
+            $bitmap.GetPixel(0, $bitmap.Height - 1),
+            $bitmap.GetPixel($bitmap.Width - 1, $bitmap.Height - 1)
+        )
+        foreach ($pixel in $cornerPixels) {
+            if ($pixel.A -gt 3) {
+                Fail "$Context must have transparent corners: $Icon"
+            }
+        }
+        $transparentSamples = 0
+        $totalSamples = 0
+        for ($y = 0; $y -lt $bitmap.Height; $y += 4) {
+            for ($x = 0; $x -lt $bitmap.Width; $x += 4) {
+                $totalSamples += 1
+                if ($bitmap.GetPixel($x, $y).A -le 3) {
+                    $transparentSamples += 1
+                }
+            }
+        }
+        if ($transparentSamples -lt [int]($totalSamples * 0.20)) {
+            Fail "$Context must have substantial transparent background: $Icon ($transparentSamples/$totalSamples sampled pixels)"
+        }
+    } catch {
+        if ($_.Exception.Message -like "$Context*") {
+            throw
+        }
+        Fail "$Context could not inspect alpha cutout data: $Icon"
+    } finally {
+        if ($null -ne $bitmap) {
+            $bitmap.Dispose()
+        }
+    }
+}
+
 function Check-Post-Battle-Dialogue($Dialogue, [string]$ScenarioId) {
     if ($null -eq $Dialogue) {
         return