shader: Add explicit location for out in gles mode

Without this, when running under ES 3.2 profile (nvidia driver), I
hit "error C5121: multiple bindings to output semantic".

Tested by running Chrome OS under qemu with these profiles:

core profile: 4.5.0 NVIDIA 384.111
core profile: 3.3 (Core Profile) Mesa 17.3.3
es profile: OpenGL ES 3.2 NVIDIA 384.111
es profile: OpenGL ES 3.0 Mesa 17.3.3
Tested-by: Jakob Bornecrantz <jakob@collabora.com>
Reviewed-by: Jakob Bornecrantz <jakob@collabora.com>

src/vrend_shader.c

@@ -2438,6 +2438,9 @@ static char *emit_ios(struct dump_ctx *ctx, char *glsl_hdr)
    }
    if (ctx->write_all_cbufs) {
       for (i = 0; i < 8; i++) {
+         if (ctx->cfg->use_gles)
+            snprintf(buf, 255, "layout (location=%d) out vec4 fsout_c%d;\n", i, i);
+         else
             snprintf(buf, 255, "out vec4 fsout_c%d;\n", i);
          STRCAT_WITH_RET(glsl_hdr, buf);
       }