direct3d: Commit dirty viewport state before clearing.

Otherwise you might have set the viewport to the full size of
the render target in SDL's API but this change hasn't been
transmitted to Direct3D yet by the time we attempt to clear.

Fixes #4210.

src/render/direct3d/SDL_render_d3d.c

@@ -1318,6 +1318,7 @@ D3D_RunCommandQueue(SDL_Renderer * renderer, SDL_RenderCommand *cmd, void *verti
                 const SDL_Rect *viewport = &data->drawstate.viewport;
                 const int backw = istarget ? renderer->target->w : data->pparams.BackBufferWidth;
                 const int backh = istarget ? renderer->target->h : data->pparams.BackBufferHeight;
+                const SDL_bool viewport_unequal = ((viewport->x == 0) && (viewport->y == 0) && (viewport->w == backw) && (viewport->h == backh)) ? SDL_FALSE : SDL_TRUE;
 
                 if (data->drawstate.cliprect_enabled || data->drawstate.cliprect_enabled_dirty) {
                     IDirect3DDevice9_SetRenderState(data->device, D3DRS_SCISSORTESTENABLE, FALSE);
@@ -1325,13 +1326,13 @@ D3D_RunCommandQueue(SDL_Renderer * renderer, SDL_RenderCommand *cmd, void *verti
                 }
 
                 /* Don't reset the viewport if we don't have to! */
-                if (!viewport->x && !viewport->y && (viewport->w == backw) && (viewport->h == backh)) {
+                if (!data->drawstate.viewport_dirty && !viewport_unequal) {
                     IDirect3DDevice9_Clear(data->device, 0, NULL, D3DCLEAR_TARGET, color, 0.0f, 0);
                 } else {
                     /* Clear is defined to clear the entire render target */
                     const D3DVIEWPORT9 wholeviewport = { 0, 0, backw, backh, 0.0f, 1.0f };
                     IDirect3DDevice9_SetViewport(data->device, &wholeviewport);
-                    data->drawstate.viewport_dirty = SDL_TRUE;
+                    data->drawstate.viewport_dirty = viewport_unequal;
                     IDirect3DDevice9_Clear(data->device, 0, NULL, D3DCLEAR_TARGET, color, 0.0f, 0);
                 }