package/ci: add a GLES3 macOS build.

There's a lot of places in the code that assume macOS is never GLES, and
with the increasingly rotten native GL support on Apple platforms people
are switching to ANGLE. So we should make sure this use case works.

package/ci/circleci.yml

@@ -406,6 +406,23 @@ jobs:
         script: unix-desktop.sh
     - lcov
 
+  macos-gles3:
+    executor: xcode-11_2
+    environment:
+      CMAKE_CXX_FLAGS: --coverage
+      CONFIGURATION: Debug
+      PLATFORM_GL_API: EGL
+      # STUPID yml interprets unquoted OFF as a boolean
+      TARGET_GLES2: "OFF"
+    steps:
+    - install-base-macos:
+        extra: sdl2 glfw wget
+    - install-swiftshader-gles:
+        build: macos-10.15
+    - build:
+        script: unix-desktop-gles.sh
+    - lcov
+
   macos-vulkan:
     # SwiftShader is built on 10.15 and can't be used on older versions due to
     # dyld: lazy symbol binding failed: Symbol not found: ____chkstk_darwin
@@ -482,6 +499,7 @@ workflows:
     - linux-threadsanitizer
     - macos-gl
     - macos-vulkan
+    - macos-gles3
     - macos-static
     - emscripten-webgl1
     - emscripten-webgl2