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.cpp

@@ -16,6 +16,11 @@ bool AffineTransform::is_identity() const
     return m_values[0] == 1 && m_values[1] == 0 && m_values[2] == 0 && m_values[3] == 1 && m_values[4] == 0 && m_values[5] == 0;
 }
 
+bool AffineTransform::is_identity_or_translation() const
+{
+    return a() == 1 && b() == 0 && c() == 0 && d() == 1;
+}
+
 static float hypotenuse(float x, float y)
 {
     // FIXME: This won't handle overflow :(

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;