[CP] [Impeller] ensure 1x1 has mipcount of 1 (#39182)

impeller/geometry/geometry_unittests.cc

@@ -646,6 +646,8 @@ TEST(GeometryTest, CanGenerateMipCounts) {
   ASSERT_EQ((Size{128, 0}.MipCount()), 1u);
   ASSERT_EQ((Size{128, -25}.MipCount()), 1u);
   ASSERT_EQ((Size{-128, 25}.MipCount()), 1u);
+  ASSERT_EQ((Size{1, 1}.MipCount()), 1u);
+  ASSERT_EQ((Size{0, 0}.MipCount()), 1u);
 }
 
 TEST(GeometryTest, CanConvertTTypesExplicitly) {

impeller/geometry/size.h

@@ -99,10 +99,12 @@ struct TSize {
   }
 
   constexpr size_t MipCount() const {
+    constexpr size_t minimum_mip = 1u;
     if (!IsPositive()) {
-      return 1u;
+      return minimum_mip;
     }
-    return std::max(ceil(log2(width)), ceil(log2(height)));
+    size_t result = std::max(ceil(log2(width)), ceil(log2(height)));
+    return std::max(result, minimum_mip);
   }
 };