DD: Refactor BlitFramebuffer.

In preparation for glBlitNamedFramebuffer, the DD table function
BlitFramebuffer needs to accept two arbitrary framebuffer objects rather
than assuming ctx->ReadBuffer and ctx->DrawBuffer.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/main/blit.c

@@ -506,7 +506,7 @@ _mesa_BlitFramebuffer(GLint srcX0, GLint srcY0, GLint srcX1, GLint srcY1,
    }
 
    ASSERT(ctx->Driver.BlitFramebuffer);
-   ctx->Driver.BlitFramebuffer(ctx,
+   ctx->Driver.BlitFramebuffer(ctx, ctx->ReadBuffer, ctx->DrawBuffer,
                                srcX0, srcY0, srcX1, srcY1,
                                dstX0, dstY0, dstX1, dstY1,
                                mask, filter);