Platform: add context release also to iOS, WGL and Windows EGL apps.

src/Magnum/Platform/WindowlessIosApplication.h

@@ -115,10 +115,23 @@ class WindowlessIosContext {
          * @brief Make the context current
          *
          * Prints error message and returns @cpp false @ce on failure,
-         * otherwise returns @cpp true @ce.
+         * otherwise returns @cpp true @ce. If the context is current on
+         * another thread, you have to @ref release() it there first --- an
+         * OpenGL context can't be current in multiple threads at the same
+         * time.
          */
         bool makeCurrent();
 
+        /**
+         * @brief Release current context
+         * @m_since_latest
+         *
+         * Releases a context previously made current using @ref makeCurrent().
+         * Prints error message and returns @cpp false @ce on failure,
+         * otherwise returns @cpp true @ce.
+         */
+        bool release();
+
         /**
          * @brief Underlying OpenGL context
          * @m_since_latest

src/Magnum/Platform/WindowlessIosApplication.mm

@@ -74,6 +74,14 @@ bool WindowlessIosContext::makeCurrent() {
     return false;
 }
 
+bool WindowlessIosContext::release() {
+    if([EAGLContext setCurrentContext:nil])
+        return true;
+
+    Error() << "Platform::WindowlessIosContext::release(): cannot release current context";
+    return false;
+}
+
 #ifndef DOXYGEN_GENERATING_OUTPUT
 WindowlessIosApplication::WindowlessIosApplication(const Arguments& arguments): WindowlessIosApplication{arguments, Configuration{}} {}
 #endif

src/Magnum/Platform/WindowlessWglApplication.cpp

@@ -243,6 +243,14 @@ bool WindowlessWglContext::makeCurrent() {
     return false;
 }
 
+bool WindowlessWglContext::release() {
+    if(wglMakeCurrent(_deviceContext, nullptr))
+        return true;
+
+    Error() << "Platform::WindowlessWglContext::release(): cannot release current context:" << GetLastError();
+    return false;
+}
+
 WindowlessWglContext::Configuration::Configuration():
     #ifndef MAGNUM_TARGET_GLES
     _flags{Flag::ForwardCompatible}

src/Magnum/Platform/WindowlessWglApplication.h

@@ -130,10 +130,23 @@ class WindowlessWglContext {
          * @brief Make the context current
          *
          * Prints error message and returns @cpp false @ce on failure,
-         * otherwise returns @cpp true @ce.
+         * otherwise returns @cpp true @ce. If the context is current on
+         * another thread, you have to @ref release() it there first --- an
+         * OpenGL context can't be current in multiple threads at the same
+         * time.
          */
         bool makeCurrent();
 
+        /**
+         * @brief Release current context
+         * @m_since_latest
+         *
+         * Releases a context previously made current using @ref makeCurrent().
+         * Prints error message and returns @cpp false @ce on failure,
+         * otherwise returns @cpp true @ce.
+         */
+        bool release();
+
         /**
          * @brief Underlying OpenGL context
          * @m_since{2020,06}

src/Magnum/Platform/WindowlessWindowsEglApplication.cpp

@@ -173,6 +173,14 @@ bool WindowlessWindowsEglContext::makeCurrent() {
     return false;
 }
 
+bool WindowlessWindowsEglContext::release() {
+    if(eglMakeCurrent(_display, EGL_NO_SURFACE, EGL_NO_SURFACE, EGL_NO_CONTEXT))
+        return true;
+
+    Error() << "Platform::WindowlessWindowsEglApplication::release(): cannot release current context:" << GetLastError();
+    return false;
+}
+
 #ifndef DOXYGEN_GENERATING_OUTPUT
 WindowlessWindowsEglApplication::WindowlessWindowsEglApplication(const Arguments& arguments): WindowlessWindowsEglApplication{arguments, Configuration{}} {}
 #endif

src/Magnum/Platform/WindowlessWindowsEglApplication.h

@@ -115,10 +115,23 @@ class WindowlessWindowsEglContext {
          * @brief Make the context current
          *
          * Prints error message and returns @cpp false @ce on failure,
-         * otherwise returns @cpp true @ce.
+         * otherwise returns @cpp true @ce. If the context is current on
+         * another thread, you have to @ref release() it there first --- an
+         * OpenGL context can't be current in multiple threads at the same
+         * time.
          */
         bool makeCurrent();
 
+        /**
+         * @brief Release current context
+         * @m_since_latest
+         *
+         * Releases a context previously made current using @ref makeCurrent().
+         * Prints error message and returns @cpp false @ce on failure,
+         * otherwise returns @cpp true @ce.
+         */
+        bool release();
+
         /**
          * @brief Underlying OpenGL context
          * @m_since{2020,06}