Make SDL_GL_GetAttribute(SDL_GL_SWAP_CONTROL) work with GLX_SGI_swap_…

…control.

  Fixes Bugzilla #697.

src/video/x11/SDL_x11gl.c

@@ -289,7 +289,9 @@ int X11_GL_CreateContext(_THIS)
 		if ( this->gl_data->glXSwapIntervalMESA ) {
 			this->gl_data->glXSwapIntervalMESA(this->gl_config.swap_control);
 		} else if ( this->gl_data->glXSwapIntervalSGI ) {
-			this->gl_data->glXSwapIntervalSGI(this->gl_config.swap_control);
+			if (this->gl_data->glXSwapIntervalSGI(this->gl_config.swap_control) != 0) {
+				this->gl_data->sgi_swap_interval = this->gl_config.swap_control;
+			}
 		}
 	}
 #else
@@ -412,7 +414,10 @@ int X11_GL_GetAttribute(_THIS, SDL_GLattr attrib, int* value)
 	    case SDL_GL_SWAP_CONTROL:
 		if ( this->gl_data->glXGetSwapIntervalMESA ) {
 			*value = this->gl_data->glXGetSwapIntervalMESA();
-			return(0);
+			return 0;
+		} else if ( this->gl_data->glXSwapIntervalSGI ) {
+			*value = this->gl_data->sgi_swap_interval;
+			return 0;
 		} else {
 			unsupported = 1;
 		}

src/video/x11/SDL_x11gl_c.h

@@ -75,6 +75,7 @@ struct SDL_PrivateGLData {
     GLint (*glXSwapIntervalMESA) ( unsigned interval );
     GLint (*glXGetSwapIntervalMESA) ( void );
 
+    int sgi_swap_interval;
 #endif /* SDL_VIDEO_OPENGL_GLX */
 };
 

src/video/x11/SDL_x11video.c

@@ -121,6 +121,8 @@ static SDL_VideoDevice *X11_CreateDevice(int devindex)
 		SDL_memset(device->hidden, 0, (sizeof *device->hidden));
 		SDL_memset(device->gl_data, 0, (sizeof *device->gl_data));
 
+		device->gl_data->sgi_swap_interval = 1;
+
 		/* Set the driver flags */
 		device->handles_any_size = 1;