mesa: Add dd_function_table::PrepareExecBegin

This hook allows the driver to prepare for a glBegin/glEnd.

i965 will use the hook to avoid avoid recursive calls to FLUSH_VERTICES
during a buffer resolve meta-op.

Detailed Justification
----------------------
When vertices are queued during a glBegin/glEnd block, those vertices must
of course be drawn before any rendering state changes. To enusure this,
Mesa calls FLUSH_VERTICES as a prehook to such state changes. Therefore,
FLUSH_VERTICES itself cannot change rendering state without falling into
a recursive trap.

This precludes meta-ops, namely i965 buffer resolves, from occuring while
any vertices are queued. To avoid that situation, i965 must satisfy the
following condition: that it queues no vertex if a buffer needs resolving.
To satisfy this, i965 will use the PrepareExecBegin hook to resolve all
buffers on entering a glBegin/glEnd block.

--------
v2: Don't add dd_function_table::CleanupExecEnd. Anholt and I discovered
    that hook to be unnecessary.

Reviewed-by: Brian Paul <brianp@vmware.com>
Signed-off-by: Chad Versace <chad@chad-versace.us>

src/mesa/drivers/common/driverfuncs.c

@@ -208,6 +208,7 @@ _mesa_init_driver_functions(struct dd_function_table *driver)
    driver->ProgramStringNotify = _tnl_program_string;
    driver->FlushVertices = NULL;
    driver->SaveFlushVertices = NULL;
+   driver->PrepareExecBegin = NULL;
    driver->NotifySaveBegin = NULL;
    driver->LightingSpaceChange = NULL;
 

src/mesa/main/dd.h

@@ -860,6 +860,14 @@ struct dd_function_table {
    void (*FlushVertices)( struct gl_context *ctx, GLuint flags );
    void (*SaveFlushVertices)( struct gl_context *ctx );
 
+   /**
+    * \brief Hook for drivers to prepare for a glBegin/glEnd block
+    *
+    * This hook is called in vbo_exec_Begin() before any action, including
+    * state updates, occurs.
+    */
+   void (*PrepareExecBegin)( struct gl_context *ctx );
+
    /**
     * Give the driver the opportunity to hook in its own vtxfmt for
     * compiling optimized display lists.  This is called on each valid

src/mesa/vbo/vbo_exec_api.c

@@ -570,6 +570,9 @@ static void GLAPIENTRY vbo_exec_Begin( GLenum mode )
          return;
       }
 
+      if (ctx->Driver.PrepareExecBegin)
+	 ctx->Driver.PrepareExecBegin(ctx);
+
       if (ctx->NewState) {
 	 _mesa_update_state( ctx );