virgl: Implement clear_render_target and clear_depth_stencil
As part of Virtio GPU 3D acceleration for windows clear_render_target and clear_depth_stencil are required. And as requested there they should be implemented as new command.
To support gallium clear_render_target and clear_depth_stencil command VIRGL_CCMD_CLEAR_SURFACE, that clears specific virgl surface, is introduced.
virglrenderer counterpart: virgl/virglrenderer!1289 (merged)