♻️ engine.h: Minor refactoring

1. Move `SetPixel` definition to the header to make it easier for the
   compiler to inline (make it inlinable even without LTO).
2. Add an `operator[](Point)` overload to `CelOutputBuffer`.

Source/engine.cpp

@@ -551,14 +551,6 @@ void CelBlitOutlineTo(const CelOutputBuffer &out, BYTE col, int sx, int sy, cons
 	}
 }
 
-void SetPixel(const CelOutputBuffer &out, Point position, BYTE col)
-{
-	if (!out.in_bounds(position))
-		return;
-
-	*out.at(position.x, position.y) = col;
-}
-
 void DrawHorizontalLine(const CelOutputBuffer &out, Point from, int width, std::uint8_t colorIndex)
 {
 	if (from.y < 0 || from.y >= out.h() || from.x >= out.w() || width <= 0 || from.x + width <= 0)
@@ -574,7 +566,7 @@ void DrawHorizontalLine(const CelOutputBuffer &out, Point from, int width, std::
 
 void UnsafeDrawHorizontalLine(const CelOutputBuffer &out, Point from, int width, std::uint8_t colorIndex)
 {
-	std::memset(out.at(from.x, from.y), colorIndex, width);
+	std::memset(&out[from], colorIndex, width);
 }
 
 void DrawVerticalLine(const CelOutputBuffer &out, Point from, int height, std::uint8_t colorIndex)
@@ -592,7 +584,7 @@ void DrawVerticalLine(const CelOutputBuffer &out, Point from, int height, std::u
 
 void UnsafeDrawVerticalLine(const CelOutputBuffer &out, Point from, int height, std::uint8_t colorIndex)
 {
-	auto *dst = out.at(from.x, from.y);
+	auto *dst = &out[from];
 	const auto pitch = out.pitch();
 	while (height-- > 0) {
 		*dst = colorIndex;

Source/engine.h

@@ -260,6 +260,11 @@ struct CelOutputBuffer {
 		return region.h;
 	}
 
+	std::uint8_t &operator[](Point p) const
+	{
+		return *at(p.x, p.y);
+	}
+
 	BYTE *at(int x, int y) const
 	{
 		return static_cast<BYTE *>(surface->pixels) + region.x + x + surface->pitch * (region.y + y);
@@ -283,7 +288,7 @@ struct CelOutputBuffer {
 		return surface->pitch;
 	}
 
-	bool in_bounds(Point position) const
+	bool InBounds(Point position) const
 	{
 		return position.x >= 0 && position.y >= 0 && position.x < region.w && position.y < region.h;
 	}
@@ -520,7 +525,13 @@ void CelBlitOutlineTo(const CelOutputBuffer &out, BYTE col, int sx, int sy, cons
  * @param point Target buffer coordinate
  * @param col Color index from current palette
  */
-void SetPixel(const CelOutputBuffer &out, Point position, BYTE col);
+inline void SetPixel(const CelOutputBuffer &out, Point position, std::uint8_t col)
+{
+	if (!out.InBounds(position))
+		return;
+
+	out[position] = col;
+}
 
 /**
  * @brief Blit CL2 sprite, to the back buffer at the given coordianates