Replace libpng submodule with source (#2023)

* Replace libpng submodule with source

Taken from 505e70834d35383537b6491e7ae8641f1a4bed1876dbfe361201fc80868d88ca  libpng-1.6.37.tar.xz

* Remove submodule-related config from CI

.github/workflows/ci.yml

@@ -8,8 +8,6 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-        with:
-          submodules: true
       - name: Install dependencies
         run: sudo apt-get update && sudo apt-get install -y
           libasound2-dev
@@ -80,8 +78,6 @@ jobs:
     runs-on: macos-latest
     steps:
       - uses: actions/checkout@v2
-        with:
-          submodules: true
       - name: Install dependencies
         run: brew install
           nasm
@@ -95,8 +91,6 @@ jobs:
     runs-on: windows-latest
     steps:
       - uses: actions/checkout@v2
-        with:
-          submodules: true
       - name: Configure
         run: cmake -B build
       - name: Build