Created attachment 2286 [details]
SDL_render_sw.c patch file

The software renderer uses the SW_RenderData struct, which has two fields: SDL_Surface* surface and SDL_Surface* window. From what I can see (based on the implementation of SW_SetRenderTarget at line 319 in SDL_render_sw.c), the surface field is treated as the current rendering target while the window field is treated as the default rendering target. To set the rendering target to a texture created with access method SDL_TEXTUREACCESS_TARGET, one would call SDL_SetRenderTarget with the texture of interest. To set the rendering target back to the default target, one would call SDL_SetRenderTarget will a NULL pointer. At line 326 in SDL_render_sw.c, the surface field gets set to the window field when a NULL pointer is passed in to the SW_SetRenderTarget method. After doing a quick search in SDL_render_sw.c I noticed that the window field never actually gets initialized anywhere (i.e. not in SW_ActivateRenderer or SW_CreateRendererForSurface). 

With the code in its current state I get the following error after running the following block of code:

'
int bits_per_pixel;
Uint32 rmask, gmask, bmask, amask;

SDL_PixelFormatEnumToMask( SDL_PIXELFORMAT_ARGB888, &bits_per_pixel, &rmask, &gmask, &bmask, &amask );

SDL_Surface* surface = SDL_CreateRGBSurface( 0, 800, 600, bits_per_pixel, rmask, gmask, bmask, amask );

SDL_Renderer* sw_renderer = SDL_CreateSoftwareRenderer( surface );

SDL_Texture* texture = SDL_CreateTexture( sw_renderer, SDL_PIXELFORMAT_ARGB8888, SDL_TEXTUREACCESS_TARGET, 800, 600 );

SDL_SetRenderTarget( sw_renderer, texture );

SDL_SetRenderDrawColor( sw_renderer, 0xFF, 0, 0, 0xFF );
SDL_RenderClear( sw_renderer );
SDL_RenderPresent( sw_renderer );

SDL_SetRenderTarget( sw_renderer, NULL );

int return_value = SDL_RenderCopy( sw_renderer, texture, NULL, NULL );

if( return_value != 0 )
  printf( "The texture could not be rendered! SDL_Error: %s\n", SDL_GetError() );
`

Output: "The texture could not be rendered! SDL error: Invalid window"

Potential Fix: By initializing the window field of the SDL_RendererData object to the input surface in the SW_CreateRendererForSurface method the default target surface will be cached and subsequent calls to SW_SetRenderTarget with a NULL texture pointer will correctly restore the default surface target. See the attached patch file for SDL_render_sw.c