Add builder for libimagequant

This is supposed to work with the v2 based msvc branch, but not the
current rust based implementation.  As such, it should probably not be
used for production purposes, but seems to be good enough for some
experiments.

Since there are apparently some OMP constructs which are not supported
by MSVC, we disable these without further ado.

.github/workflows/libimagequant_msvc.yml

@@ -0,0 +1,49 @@
+name: Build libimagequant_msvc
+on:
+  workflow_dispatch:
+    inputs:
+      version:
+        description: libimagequant tag to build
+        required: true
+      php:
+        description: PHP version to build for
+        required: true
+defaults:
+  run:
+    shell: cmd
+jobs:
+  build:
+    strategy:
+      matrix:
+          arch: [x64, x86]
+    runs-on: windows-2022
+    steps:
+      - name: Checkout winlib-builder
+        uses: actions/checkout@v4
+        with:
+          path: winlib-builder
+      - name: Checkout libimagequant
+        uses: actions/checkout@v4
+        with:
+          path: libimagequant
+          repository: ImageOptim/libimagequant
+          ref: ${{github.event.inputs.version}}
+      - name: Patch libimagequant
+        run: cd libimagequant && git apply --ignore-whitespace ..\winlib-builder\patches\libimagequant_msvc.patch
+      - name: Compute virtual inputs
+        id: virtuals
+        run: powershell winlib-builder/scripts/compute-virtuals -version ${{github.event.inputs.php}} -arch ${{matrix.arch}}
+      - name: Configure libimagequant
+        run: cd libimagequant && md build && cd build && cmake -G "Visual Studio 17 2022" -A ${{steps.virtuals.outputs.msarch}} -T ${{steps.virtuals.outputs.msts}} -DCMAKE_SYSTEM_VERSION=${{steps.virtuals.outputs.winsdk}}--install-prefix ${{github.workspace}}\install ..
+      - name: Build libimagequant
+        run: cd libimagequant\build && cmake --build . --config RelWithDebInfo
+      - name: Install freetype
+        run: |
+          cd libimagequant\build
+          cmake --install . --config RelWithDebInfo
+          xcopy RelWithDebInfo\imagequant.pdb ..\..\install\bin\*
+      - name: Upload artifacts
+        uses: actions/upload-artifact@v4
+        with:
+          name: ${{github.event.inputs.version}}-${{steps.virtuals.outputs.vs}}-${{matrix.arch}}
+          path: install

patches/libimagequant_msvc.patch

@@ -0,0 +1,51 @@
+ CMakeLists.txt | 4 ++--
+ mediancut.c    | 6 +++++-
+ 2 files changed, 7 insertions(+), 3 deletions(-)
+
+diff --git a/CMakeLists.txt b/CMakeLists.txt
+index d14f058..46c7ef8 100644
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -52,5 +52,5 @@ configure_file(imagequant.pc.in imagequant.pc @ONLY)
+
+ install(TARGETS imagequant        LIBRARY           DESTINATION ${LIB_INSTALL_DIR})
+ install(FILES                     libimagequant.h   DESTINATION include)
+-install(FILES ${CMAKE_BINARY_DIR}/libimagequant_a.a DESTINATION ${LIB_INSTALL_DIR} RENAME libimagequant.a)
+-install(FILES ${CMAKE_BINARY_DIR}/imagequant.pc     DESTINATION ${LIB_INSTALL_DIR}/pkgconfig)
++# install(FILES ${CMAKE_BINARY_DIR}/libimagequant_a.a DESTINATION lib RENAME libimagequant.a)
++install(FILES ${CMAKE_BINARY_DIR}/imagequant.pc     DESTINATION lib/pkgconfig)
+diff --git a/mediancut.c b/mediancut.c
+index 2df7941..21b38cf 100644
+--- a/mediancut.c
++++ b/mediancut.c
+@@ -200,7 +200,7 @@ static double prepare_sort(struct box *b, hist_item achv[])
+ #if __GNUC__ >= 9 || __clang__
+     #pragma omp parallel for if (colors > 25000) \
+         schedule(static) default(none) shared(achv, channels, colors, ind1)
+-#else
++#elif !defined(_MSC_VER)
+     #pragma omp parallel for if (colors > 25000) \
+         schedule(static) default(none) shared(achv, channels)
+ #endif
+@@ -217,8 +217,10 @@ static double prepare_sort(struct box *b, hist_item achv[])
+     // box will be split to make color_weight of each side even
+     const unsigned int ind = b->ind, end = ind+b->colors;
+     double totalvar = 0;
++#ifndef _MSC_VER
+     #pragma omp parallel for if (end - ind > 15000) \
+         schedule(static) default(shared) reduction(+:totalvar)
++#endif
+     for(unsigned int j=ind; j < end; j++) totalvar += (achv[j].color_weight = color_weight(median, achv[j]));
+     return totalvar / 2.0;
+ }
+@@ -439,8 +441,10 @@ static f_pixel averagepixels(unsigned int clrs, const hist_item achv[])
+ {
+     double r = 0, g = 0, b = 0, a = 0, sum = 0;
+
++#ifndef _MSC_VER
+     #pragma omp parallel for if (clrs > 25000) \
+         schedule(static) default(shared) reduction(+:a) reduction(+:r) reduction(+:g) reduction(+:b) reduction(+:sum)
++#endif
+     for(unsigned int i = 0; i < clrs; i++) {
+         const f_pixel px = achv[i].acolor;
+         const double weight = achv[i].adjusted_weight;