Add texture alpha blending

sdlpp/sdlpp_texture.cpp

@@ -73,4 +73,9 @@ void Texture::setTextureFromSurface( std::shared_ptr< Renderer > &renderer,
     }
     SDL_FreeSurface( surface );
 }
+
+void Texture::setAlpha( uint8_t alpha ) {
+    SDL_SetTextureBlendMode( texture, SDL_BLENDMODE_BLEND );
+    SDL_SetTextureAlphaMod( texture, alpha );
+}
 } // namespace SDLPP