svga: Fix zslice index to svga_texture_copy_handle_resource()

The zslice index to svga_texture_copy_handle_resource() is not adjusted
and should be a signed integer.

This patch fixes piglit tests for non-vgpu10 including
   spec@arb_framebuffer_object@fbo-generatemipmap-3d
   spec@glsl-1.20@execution@tex-miplevel-selection gl2:texture* 3d

Tested with MTT piglit and glretrace
1 file changed