Added a test for assembling the GL interface

tests/Tests/GRGlInterfaceTest.cs

@@ -20,5 +20,57 @@ namespace SkiaSharp.Tests
 				Assert.True(glInterface.Validate());
 			}
 		}
+
+		[Test]
+		public void AssembleInterfaceIsValid()
+		{
+			using (var ctx = CreateGlContext()) {
+				ctx.MakeCurrent();
+
+				if (IsMac) {
+					var lib = MacDynamicLibraries.dlopen("/System/Library/Frameworks/OpenGL.framework/Versions/A/Libraries/libGL.dylib", 1);
+
+					var glInterface = GRGlInterface.AssembleGlInterface((context, name) => {
+						return MacDynamicLibraries.dlsym(lib, name);
+					});
+
+					Assert.NotNull(glInterface);
+					Assert.True(glInterface.Validate());
+
+					MacDynamicLibraries.dlclose(lib);
+				} else if (IsWindows) {
+					var lib = WindowsDynamicLibraries.LoadLibrary("opengl32.dll");
+
+					var glInterface = GRGlInterface.AssembleGlInterface((context, name) => {
+						var ptr = WindowsDynamicLibraries.GetProcAddress(lib, name);
+						if (ptr == IntPtr.Zero) {
+							ptr = wglGetProcAddress(name);
+						}
+						return ptr;
+					});
+
+					Assert.NotNull(glInterface);
+					Assert.True(glInterface.Validate());
+
+					WindowsDynamicLibraries.FreeLibrary(lib);
+				} else if (IsLinux) {
+					var lib = LinuxDynamicLibraries.dlopen("libGL.so.1", 1);
+
+					var glInterface = GRGlInterface.AssembleGlInterface((context, name) => {
+						return LinuxDynamicLibraries.dlsym(lib, name);
+					});
+
+					Assert.NotNull(glInterface);
+					Assert.True(glInterface.Validate());
+
+					LinuxDynamicLibraries.dlclose(lib);
+				} else {
+					// more platforms !!!
+				}
+			}
+		}
+
+		[DllImport("opengl32.dll", CallingConvention = CallingConvention.Winapi)]
+		public static extern IntPtr wglGetProcAddress([MarshalAs(UnmanagedType.LPStr)] string lpszProc);
 	}
 }

tests/Tests/SKTest.cs

@@ -53,6 +53,39 @@ namespace SkiaSharp.Tests
 		protected static bool IsWindows => !IsUnix;
 #endif
 
+		public static class MacDynamicLibraries
+		{
+			private const string SystemLibrary = "/usr/lib/libSystem.dylib";
+			[DllImport(SystemLibrary)]
+			public static extern IntPtr dlopen(string path, int mode);
+			[DllImport(SystemLibrary)]
+			public static extern IntPtr dlsym(IntPtr handle, string symbol);
+			[DllImport(SystemLibrary)]
+			public static extern void dlclose(IntPtr handle);
+		}
+
+		public static class LinuxDynamicLibraries
+		{
+			private const string SystemLibrary = "libdl.so";
+			[DllImport(SystemLibrary)]
+			public static extern IntPtr dlopen(string path, int mode);
+			[DllImport(SystemLibrary)]
+			public static extern IntPtr dlsym(IntPtr handle, string symbol);
+			[DllImport(SystemLibrary)]
+			public static extern void dlclose(IntPtr handle);
+		}
+
+		public static class WindowsDynamicLibraries
+		{
+			private const string SystemLibrary = "Kernel32.dll";
+			[DllImport (SystemLibrary, SetLastError = true, CharSet = CharSet.Ansi)]
+			public static extern IntPtr LoadLibrary(string lpFileName);
+			[DllImport (SystemLibrary, SetLastError = true, CharSet = CharSet.Ansi)]
+			public static extern IntPtr GetProcAddress(IntPtr hModule, string lpProcName);
+			[DllImport (SystemLibrary, SetLastError = true, CharSet = CharSet.Ansi)]
+			public static extern void FreeLibrary(IntPtr hModule);
+		}
+
 		protected GlContext CreateGlContext()
 		{
 			if (IsLinux) {