vrend: always set CLAMP_READ_COLOR to false in do_readpixels

Some paths don't emit rasterizer state or framebuffer state before calling
do_readpixels, and because of that the clamping is not done properly.

Fixes 0d84775435
Fixes several piglit snorm tests.

Signed-off-by: Italo Nicola <italonicola@collabora.com>
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>

src/vrend_renderer.c

@@ -5805,11 +5805,6 @@ static void vrend_hw_emit_rs(struct vrend_context *ctx)
          report_core_warn(ctx, CORE_PROFILE_WARN_CLAMP);
    }
 
-   /* read-color-clamping is handled in the mesa frontend */
-   if (!vrend_state.use_gles) {
-       glClampColor(GL_CLAMP_READ_COLOR_ARB, GL_FALSE);
-   }
-
    if (has_feature(feat_multisample)) {
       if (has_feature(feat_sample_mask)) {
 	 if (state->multisample)
@@ -8073,6 +8068,11 @@ static void do_readpixels(struct vrend_resource *res,
       }
    }
 
+   /* read-color clamping is handled in the mesa frontend */
+   if (!vrend_state.use_gles) {
+       glClampColor(GL_CLAMP_READ_COLOR_ARB, GL_FALSE);
+   }
+
    if (has_feature(feat_arb_robustness))
       glReadnPixelsARB(x, y, width, height, format, type, bufSize, data);
    else if (has_feature(feat_gles_khr_robustness))