mesa: move gl_texture_image::FetchTexel fields to swrast

This also involves passing swrast_texture_image instead of gl_texture_image
into all the fetch functions.

src/mesa/main/teximage.c

@@ -1076,8 +1076,6 @@ clear_teximage_fields(struct gl_texture_image *img)
    img->DepthLog2 = 0;
    img->Data = NULL;
    img->TexFormat = MESA_FORMAT_NONE;
-   img->FetchTexelc = NULL;
-   img->FetchTexelf = NULL;
 }