it: avoid problematic halfway colors

.builds/test.yml

@@ -16,5 +16,4 @@ tasks:
   - test: |
       cd jay
       export RUST_BACKTRACE=1
-      export GALLIUM_DRIVER=softpipe
       ./target/debug/jay run-tests

src/it/tests/t0039_alpha_modifier.rs

@@ -45,9 +45,9 @@ async fn test(run: Rc<TestRun>) -> TestResult {
 
     client.compare_screenshot("1", false).await?;
 
-    alpha1.set_multiplier(0.5)?;
+    alpha1.set_multiplier(0.51)?;
     ss1.commit()?;
-    alpha2.set_multiplier(0.5)?;
+    alpha2.set_multiplier(0.51)?;
     ss2.commit()?;
 
     client.compare_screenshot("2", false).await?;