downgrade to Qt 5.12.10

More up-to-date versions of Qt have major performance problems
with QGLWidget. This will not be fixed for Mixxx 2.3:
mixxxdj/mixxx#1974

scripts/macosx/build_environment.sh

@@ -16,7 +16,7 @@ MIXXX_MACOSX_SDK=$(xcodebuild -version -sdk macosx SDKVersion)
 # Qt 5.11 requires a minimum of macOS 10.11.
 MIXXX_MACOSX_TARGET='10.11'
 MIXXX_MACOSX_STDLIB='libc++'
-MIXXX_QT_VERSION='5.14.2'
+MIXXX_QT_VERSION='5.12.10'
 ENABLE_I386=false
 ENABLE_X86_64=false
 ENABLE_PPC=false

scripts/macosx/download_dependencies.sh

@@ -51,7 +51,7 @@ download_and_verify http://www.portaudio.com/archives/pa_stable_v190600_20161030
 download_and_verify http://downloads.sourceforge.net/project/portmedia/portmidi/217/portmidi-src-217.zip 08e9a892bd80bdb1115213fb72dc29a7bf2ff108b378180586aa65f3cfd42e0f
 download_and_verify https://github.com/google/protobuf/archive/v2.6.1.tar.gz 2667b7cda4a6bc8a09e5463adf3b5984e08d94e72338277affa8594d8b6e5cd1 protobuf-2.6.1.tar.gz
 #download_and_verify https://download.qt.io/official_releases/qt/4.8/4.8.7/qt-everywhere-opensource-src-4.8.7.tar.gz e2882295097e47fe089f8ac741a95fef47e0a73a3f3cdf21b56990638f626ea0
-download_and_verify https://download.qt.io/archive/qt/5.14/5.14.2/single/qt-everywhere-src-5.14.2.tar.xz c6fcd53c744df89e7d3223c02838a33309bd1c291fcb6f9341505fe99f7f19fa
+download_and_verify https://download.qt.io/archive/qt/5.12/5.12.10/single/qt-everywhere-src-5.12.10.tar.xz 3e0ee1e57f5cf3eeb038d0b4b22c7eb442285c62639290756b39dc93a1d0e14f
 download_and_verify http://code.breakfastquay.com/attachments/download/34/rubberband-1.8.1.tar.bz2 ff0c63b0b5ce41f937a8a3bc560f27918c5fe0b90c6bc1cb70829b86ada82b75
 download_and_verify http://downloads.xiph.org/releases/libshout/libshout-2.4.1.tar.gz f3acb8dec26f2dbf6df778888e0e429a4ce9378a9d461b02a7ccbf2991bbf24d
 download_and_verify https://downloads.sourceforge.net/project/libshoutidjc.idjc.p/libshout-idjc-2.4.1.tar.gz 4751c75fc85fc5d10e5b03753b046bcdee39576278bf30565c751816a87facdf