gallium: avoid mapping same vertex buffer in subsequent frames

Quite a few util modules were maintaining a single vertex buffer over multiple
frames, and potentially reusing it in subsequent frames.  Unfortunately that
would force us into syncrhonous rendering as the buffer manager would be
forced to wait for the previous rendering to complete prior to allowing the
map.

This resolves that issue, but requires the state tracker to issue a few new
flush() calls at the end of each frame.

src/mesa/state_tracker/st_cb_readpixels.c

@@ -317,10 +317,8 @@ st_readpixels(GLcontext *ctx, GLint x, GLint y, GLsizei width, GLsizei height,
    if (!dest)
       return;
 
-   st_flush_bitmap_cache(ctx->st);
-
    /* make sure rendering has completed */
-   pipe->flush(pipe, PIPE_FLUSH_RENDER_CACHE, NULL);
+   st_flush(ctx->st, PIPE_FLUSH_RENDER_CACHE, NULL);
 
    if (format == GL_STENCIL_INDEX) {
       st_read_stencil_pixels(ctx, x, y, width, height, type, pack, dest);