[core] allow context to be abandoned on destruction

- This enables the use case where the system already destroyed the underlying GL resources. Otherwise, the cleanup would fail and crash

src/mbgl/gl/context.cpp

@@ -87,7 +87,9 @@ static_assert(std::is_same<BinaryProgramFormat, GLenum>::value, "OpenGL type mis
 Context::Context() = default;
 
 Context::~Context() {
-    reset();
+    if (cleanupOnDestruction) {
+        reset();
+    }
 }
 
 void Context::initializeExtensions(const std::function<gl::ProcAddress(const char*)>& getProcAddress) {

src/mbgl/gl/context.hpp

@@ -190,7 +190,13 @@ public:
         return vertexArray.get();
     }
 
+    void setCleanupOnDestruction(bool cleanup) {
+        cleanupOnDestruction = cleanup;
+    }
+
 private:
+    bool cleanupOnDestruction = true;
+
     std::unique_ptr<extension::Debugging> debugging;
     std::unique_ptr<extension::VertexArray> vertexArray;
 #if MBGL_HAS_BINARY_PROGRAMS