Revert "testautomation: don't validate alpha values on XRGB formats"

This reverts commit 759e01bd64.

It's better to return the expected format from SDL_RenderReadPixels() than skip alpha testing.

test/testautomation_render.c

@@ -897,12 +897,7 @@ static void testBlendModeOperation(TestRenderOperation op, int mode, SDL_PixelFo
     deltaR = SDL_abs((int)actualR - expectedR);
     deltaG = SDL_abs((int)actualG - expectedG);
     deltaB = SDL_abs((int)actualB - expectedB);
-    if (SDL_ISPIXELFORMAT_ALPHA(dst_format)) {
-        deltaA = SDL_abs((int)actualA - expectedA);
-    } else {
-        // The alpha channel is ignored in non-alpha formats, so don't validate it
-        deltaA = 0;
-    }
+    deltaA = SDL_abs((int)actualA - expectedA);
     SDLTest_AssertCheck(
         deltaR <= MAXIMUM_ERROR &&
         deltaG <= MAXIMUM_ERROR &&