[Impeller] dont generate final 1x1 mip level to work around Adreno GPU bug (#161192)

Generating the final 1x1 mip level of a texture causes the entire
texture to become corrupted on some Adreno GPUs.

flutter/flutter#160441
flutter/flutter#159876
flutter/flutter#160587

Author: Jonah Williams

engine/src/flutter/impeller/geometry/geometry_unittests.cc

@@ -578,11 +578,11 @@ TEST(GeometryTest, QuaternionVectorMultiply) {
 }
 
 TEST(GeometryTest, CanGenerateMipCounts) {
-  ASSERT_EQ((Size{128, 128}.MipCount()), 7u);
-  ASSERT_EQ((Size{128, 256}.MipCount()), 8u);
-  ASSERT_EQ((Size{128, 130}.MipCount()), 8u);
-  ASSERT_EQ((Size{128, 257}.MipCount()), 9u);
-  ASSERT_EQ((Size{257, 128}.MipCount()), 9u);
+  ASSERT_EQ((Size{128, 128}.MipCount()), 6u);
+  ASSERT_EQ((Size{128, 256}.MipCount()), 7u);
+  ASSERT_EQ((Size{128, 130}.MipCount()), 7u);
+  ASSERT_EQ((Size{128, 257}.MipCount()), 8u);
+  ASSERT_EQ((Size{257, 128}.MipCount()), 8u);
   ASSERT_EQ((Size{128, 0}.MipCount()), 1u);
   ASSERT_EQ((Size{128, -25}.MipCount()), 1u);
   ASSERT_EQ((Size{-128, 25}.MipCount()), 1u);

engine/src/flutter/impeller/geometry/size.h

@@ -131,12 +131,25 @@ struct TSize {
                  static_cast<Type>(std::ceil(other.height))};
   }
 
+  /// Return the mip count of the texture.
+  ///
+  /// Note: does not count the final 1x1 mip level, both for practical reasons
+  /// and to workaround driver bugs.
   constexpr size_t MipCount() const {
     constexpr size_t minimum_mip = 1u;
     if (IsEmpty()) {
       return minimum_mip;
     }
     size_t result = std::max(ceil(log2(width)), ceil(log2(height)));
+    // This check avoids creating 1x1 mip levels, which are both pointless
+    // and cause rendering problems on some Adreno GPUs.
+    // See:
+    //      * https://github.com/flutter/flutter/issues/160441
+    //      * https://github.com/flutter/flutter/issues/159876
+    //      * https://github.com/flutter/flutter/issues/160587
+    if (result > 1) {
+      result -= 1;
+    }
     return std::max(result, minimum_mip);
   }
 };