Make a few constructors explicit

This makes them allign with the other backend ctors.

Author: jjuhl

include/TGUI/Backend/Renderer/Raylib/CanvasRaylib.hpp

@@ -82,7 +82,7 @@ namespace tgui
         /// @param initRenderer Should the renderer be initialized? Should be true unless a derived class initializes it.
         /// @see create
         ////////////////////////////////////////////////////////////////////////////////////////////////////////////////
-        CanvasRaylib(const char* typeName = StaticWidgetType, bool initRenderer = true);
+        explicit CanvasRaylib(const char* typeName = StaticWidgetType, bool initRenderer = true);
 
         ////////////////////////////////////////////////////////////////////////////////////////////////////////////////
         /// @brief Copy constructor

include/TGUI/Backend/Renderer/SDL_GPU/CanvasSDLGPU.hpp

@@ -73,7 +73,7 @@ namespace tgui
         /// @param initRenderer Should the renderer be initialized? Should be true unless a derived class initializes it.
         /// @see create
         ////////////////////////////////////////////////////////////////////////////////////////////////////////////////
-        CanvasSDLGPU(const char* typeName = StaticWidgetType, bool initRenderer = true);
+        explicit CanvasSDLGPU(const char* typeName = StaticWidgetType, bool initRenderer = true);
 
         ////////////////////////////////////////////////////////////////////////////////////////////////////////////////
         /// @brief Copy constructor

include/TGUI/Backend/Renderer/SDL_Renderer/CanvasSDL.hpp

@@ -77,7 +77,7 @@ namespace tgui
         /// @param initRenderer Should the renderer be initialized? Should be true unless a derived class initializes it.
         /// @see create
         ////////////////////////////////////////////////////////////////////////////////////////////////////////////////
-        CanvasSDL(const char* typeName = StaticWidgetType, bool initRenderer = true);
+        explicit CanvasSDL(const char* typeName = StaticWidgetType, bool initRenderer = true);
 
         ////////////////////////////////////////////////////////////////////////////////////////////////////////////////
         /// @brief Copy constructor