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.
dac19f17f3