Allow draw::set_render_target to switch to the main render buffer.

This is done by passing it an empty texture.

src/draw.cpp

@@ -577,7 +577,7 @@ draw::render_target_setter::render_target_setter(const texture& t)
 	, viewport_()
 {
 	// Validate we can render to this texture.
-	assert(t.get_access() == SDL_TEXTUREACCESS_TARGET);
+	assert(!t || t.get_access() == SDL_TEXTUREACCESS_TARGET);
 
 	if (!renderer()) {
 		WRN_D << "can't set render target with null renderer";
@@ -587,7 +587,11 @@ draw::render_target_setter::render_target_setter(const texture& t)
 	target_ = video::get_render_target();
 	SDL_RenderGetViewport(renderer(), &viewport_);
 
-	video::force_render_target(t);
+	if (t) {
+		video::force_render_target(t);
+	} else {
+		video::reset_render_target();
+	}
 }
 
 draw::render_target_setter::~render_target_setter()
@@ -602,7 +606,11 @@ draw::render_target_setter::~render_target_setter()
 
 draw::render_target_setter draw::set_render_target(const texture& t)
 {
-	DBG_D << "setting render target to "
+	if (t) {
-	      << t.w() << 'x' << t.h() << " texture";
+		DBG_D << "setting render target to "
+		      << t.w() << 'x' << t.h() << " texture";
+	} else {
+		DBG_D << "setting render target to main render buffer";
+	}
 	return draw::render_target_setter(t);
 }

src/draw.hpp

@@ -446,7 +446,9 @@ private:
  * SDL_TEXTUREACCESS_TARGET access mode.
  *
  * @param t     The new render target. This must be a texture created
- *              with SDL_TEXTUREACCESS_TARGET.
+ *              with SDL_TEXTUREACCESS_TARGET, or an empty texture.
+ *              If empty, it will set the render target to Wesnoth's
+ *              primary render buffer.
  * @returns     A render_target_setter object. When this object is
  *              destroyed the render target will be restored to
  *              whatever it was before this call.

src/video.cpp

@@ -522,6 +522,11 @@ void clear_render_target()
 	force_render_target({});
 }
 
+void reset_render_target()
+{
+	force_render_target(render_texture_);
+}
+
 texture get_render_target()
 {
 	// This should always be up-to-date, but assert for sanity.
@@ -564,7 +569,7 @@ void render_screen()
 	SDL_RenderPresent(*window);
 
 	// Reset the render target to the render texture.
-	force_render_target(render_texture_);
+	reset_render_target();
 }
 
 surface read_pixels(SDL_Rect* r)

src/video.hpp

@@ -290,6 +290,9 @@ void force_render_target(const texture& t);
 /** Reset the render target to the main window / screen. */
 void clear_render_target();
 
+/** Reset the render target to the primary render buffer. */
+void reset_render_target();
+
 /** Get the current render target.
  *
  * Will return an empty texture if the render target is the underlying