LibGfx: Add AffineTransform::is_identity_or_translation()

This will allow us to implement fast paths in cases where a simple
translation can be handled faster than a full affine transform.

Userland/Libraries/LibGfx/AffineTransform.h

@@ -25,6 +25,7 @@ public:
     }
 
     bool is_identity() const;
+    bool is_identity_or_translation() const;
 
     void map(float unmapped_x, float unmapped_y, float& mapped_x, float& mapped_y) const;