glsl: fix link failure for variable-indexed varying output arrays

glsl: fix link failure for variable-indexed varying output arrays

If the vertex shader writes to a varying array with a variable index,
mark all the elements of that array as being written.

For example, if the vertex shader does:

   for (i = 0; i < 4; i++)
      gl_TexCoord[i] = expr;

Mark all texcoord outputs as being written, not just the first.

Linking will fail if a fragment shader tries to read an input that's not
written by the vertex shader.  Before this fix, this linker test could fail.