Minor fixes for C++20 compatibility

[jiahaog]

Fixes the following errors when building with a C++20 toolchain.

I don't really know C++ that well so any pointers would be appreciated.

For the change to the lambda, we can't follow the recommendation to capture *this explicitly because that wouldn't compile under C++17. This PR changes the semantics slightly to capture surface_ by reference, which might be still okay?

For b/289776142

error: implicit capture of 'this' with a capture default of '=' is deprecated [-Werror,-Wdeprecated-this-capture]
   93 |       if (surface_) {
      |           ^
note: add an explicit capture of 'this' to capture '*this' by reference
   91 |     raster_thread_merger_->SetMergeUnmergeCallback([=]() {
      |                                                     ^
      |                                                      , this

error: bitwise operation between different enumeration types ('CGImageAlphaInfo' and '(unnamed enum at third_party/apple_sdks/xcode_14_2/iphonesimulator/System/Library/Frameworks/CoreGraphics.framework/Headers/CGImage.h:53:9)') is deprecated [-Werror,-Wdeprecated-anon-enum-enum-conversion]
  159 |       static_cast<CGBitmapInfo>(kCGImageAlphaPremultipliedLast |
      |                                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^
  160 |                                 kCGBitmapByteOrder32Big),  // CGBitmapInfo bitmapInfo
      |                                 ~~~~~~~~~~~~~~~~~~~~~~~

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the [CLA].
• All existing and new tests are passing.

[jason-simmons]

Change the capture to [this]