common: ensure const equality operators

A minor syntactic adjustment to two instances of operator== and one instance of operator!=, both to the end of ensuring these locally-scoped functions are properly identified as const. In the majority of cases, this shouldn't have any impact; however, this change makes these operators play nice in c++20 contexts.

src/renderer/gl_engine/tvgGlGeometry.h

@@ -108,14 +108,14 @@ public:
         return *this;
     }
 
-    bool operator== (const GlPoint& rhs)
+    bool operator== (const GlPoint& rhs) const
     {
         if (&rhs == this) return true;
         if (rhs.x == this->x && rhs.y == this->y) return true;
         return false;
     }
 
-    bool operator!= (const GlPoint& rhs)
+    bool operator!= (const GlPoint& rhs) const
     {
         if (&rhs == this) return true;
         if (rhs.x != this->x || rhs.y != this->y) return true;

src/renderer/tvgRender.h

@@ -103,7 +103,7 @@ struct RenderRegion
     void intersect(const RenderRegion& rhs);
     void add(const RenderRegion& rhs);
 
-    bool operator==(const RenderRegion& rhs)
+    bool operator==(const RenderRegion& rhs) const
     {
         if (x == rhs.x && y == rhs.y && w == rhs.w && h == rhs.h) return true;
         return false;