SDL sub-pixel rendering support

Replaces drawing functions in `SDLPainter` with their `float` equivalents.

Fixes #2422.

src/math/rect.hpp

@@ -98,6 +98,11 @@ class Rect final
             top <= other.top && other.bottom <= bottom);
   }
 
+  int get_left() const { return left; }
+  int get_right() const { return right; }
+  int get_top() const { return top; }
+  int get_bottom() const { return bottom; }
+
   int get_width()  const { return right - left; }
   int get_height() const { return bottom - top; }
   Size get_size() const { return Size(right - left, bottom - top); }

src/math/rectf.hpp

@@ -20,13 +20,14 @@
 #include <assert.h>
 #include <iosfwd>
 
+#include <SDL.h>
+
 #include "math/anchor_point.hpp"
+#include "math/rect.hpp"
 #include "math/sizef.hpp"
 #include "math/vector.hpp"
 #include "util/log.hpp"
 
-class Rect;
-
 class Rectf final
 {
 public:
@@ -69,6 +70,12 @@ class Rectf final
     initialize();
   }
 
+  Rectf(const SDL_FRect& rect) :
+    m_p1(rect.x, rect.y), m_size(rect.w, rect.h)
+  {
+    initialize();
+  }
+
   Rectf(const Rect& rect);
 
   bool operator==(const Rectf& other) const
@@ -125,6 +132,12 @@ class Rectf final
   }
   Sizef get_size() const { return m_size; }
 
+  bool empty() const
+  {
+    return get_width() <= 0 ||
+           get_height() <= 0;
+  }
+
   void move(const Vector& v) { m_p1 += v; }
   Rectf moved(const Vector& v) const { return Rectf(m_p1 + v, m_size); }
 
@@ -179,6 +192,17 @@ class Rectf final
                    p.y - m_p1.y);
   }
 
+  Rect to_rect() const
+  {
+    return { static_cast<int>(m_p1.x), static_cast<int>(m_p1.y),
+             static_cast<int>(m_size.width), static_cast<int>(m_size.height) };
+  }
+
+  SDL_FRect to_sdl() const
+  {
+    return { m_p1.x, m_p1.y, m_size.width, m_size.height };
+  }
+
 private:
   /// upper left edge
   Vector m_p1;