st/mesa: use new _mesa_base_tex_image() helper

This involved adding a new st_texture_image_const() helper also.

Reviewed-by: Eric Anholt <eric@anholt.net>

src/mesa/state_tracker/st_atom_sampler.c

@@ -36,6 +36,7 @@
 #include "main/mtypes.h"
 #include "main/glformats.h"
 #include "main/samplerobj.h"
+#include "main/teximage.h"
 #include "main/texobj.h"
 
 #include "st_context.h"
@@ -141,7 +142,7 @@ convert_sampler(struct st_context *st,
       texobj = _mesa_get_fallback_texture(ctx, TEXTURE_2D_INDEX);
    }
 
-   teximg = texobj->Image[0][texobj->BaseLevel];
+   teximg = _mesa_base_tex_image_const(texobj);
    texBaseFormat = teximg ? teximg->_BaseFormat : GL_RGBA;
 
    msamp = _mesa_get_samplerobj(ctx, texUnit);