GraphicsBitmap: Add non-const overload of bits().

This is clearly useful, I don't know why I thought it wasn't.

SharedGraphics/GraphicsBitmap.h

@@ -23,6 +23,7 @@ public:
     RGBA32* scanline(int y);
     const RGBA32* scanline(int y) const;
 
+    byte* bits(int y);
     const byte* bits(int y) const;
 
     Rect rect() const { return { {}, m_size }; }
@@ -86,3 +87,8 @@ inline const byte* GraphicsBitmap::bits(int y) const
 {
     return reinterpret_cast<const byte*>(scanline(y));
 }
+
+inline byte* GraphicsBitmap::bits(int y)
+{
+    return reinterpret_cast<byte*>(scanline(y));
+}