mesa: allow buffers to be mapped multiple times

OpenGL allows a buffer to be mapped only once, but we also map buffers
internally, e.g. in the software primitive restart fallback, for PBOs,
vbo_get_minmax_index, etc. This has always been a problem, but it will
be a bigger problem with persistent buffer mappings, which will prevent
all Mesa functions from mapping buffers for internal purposes.

This adds a driver interface to core Mesa which supports multiple buffer
mappings and allows 2 mappings: one for the GL user and one for Mesa.

Note that Gallium supports an unlimited number of buffer and texture
mappings, so it's not really an issue for Gallium.

v2: fix unmapping in xm_dd.c, remove the GL errors there
v3: fix the intel driver (by Fredrik)

Reviewed-by: Fredrik Höglund <fredrik@kde.org>

src/mesa/main/shared.c

@@ -186,10 +186,8 @@ delete_bufferobj_cb(GLuint id, void *data, void *userData)
 {
    struct gl_buffer_object *bufObj = (struct gl_buffer_object *) data;
    struct gl_context *ctx = (struct gl_context *) userData;
-   if (_mesa_bufferobj_mapped(bufObj)) {
-      ctx->Driver.UnmapBuffer(ctx, bufObj);
-      bufObj->Pointer = NULL;
-   }
+
+   _mesa_buffer_unmap_all_mappings(ctx, bufObj);
    _mesa_reference_buffer_object(ctx, &bufObj, NULL);
 }