swrast: Drop the global mapping of buffers across glReadPixels().

Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/swrast/s_readpix.c

@@ -484,12 +484,6 @@ _swrast_ReadPixels( struct gl_context *ctx,
    if (ctx->NewState)
       _mesa_update_state(ctx);
 
-   /* Need to do swrast_render_start() before clipping or anything else
-    * since this is where a driver may grab the hw lock and get an updated
-    * window size.
-    */
-   swrast_render_start(ctx);
-
    /* Do all needed clipping here, so that we can forget about it later */
    if (_mesa_clip_readpixels(ctx, &x, &y, &width, &height, &clippedPacking)) {
 
@@ -518,6 +512,4 @@ _swrast_ReadPixels( struct gl_context *ctx,
          _mesa_unmap_pbo_dest(ctx, &clippedPacking);
       }
    }
-
-   swrast_render_finish(ctx);
 }