[WindowLessApps]: add release option to contexts [egl,cgl,glx for linux]

src/Magnum/Platform/WindowlessCglApplication.cpp

@@ -97,6 +97,14 @@ bool WindowlessCglContext::makeCurrent() {
     return false;
 }
 
+bool WindowlessCglContext::release() {
+    if(CGLSetCurrentContext(0) == kCGLNoError)
+        return true;
+
+    Error() << "Platform::WindowlessCglContext::release(): cannot release current context";
+    return false;
+}
+
 #ifndef DOXYGEN_GENERATING_OUTPUT
 WindowlessCglApplication::WindowlessCglApplication(const Arguments& arguments): WindowlessCglApplication{arguments, Configuration{}} {}
 #endif

src/Magnum/Platform/WindowlessCglApplication.h

@@ -126,6 +126,14 @@ class WindowlessCglContext {
          */
         bool makeCurrent();
 
+        /**
+         * @brief Release current context
+         *
+         * 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/WindowlessEglApplication.cpp

@@ -503,7 +503,15 @@ bool WindowlessEglContext::makeCurrent() {
         return true;
     #endif
 
-    Error() << "Platform::WindowlessEglApplication::tryCreateContext(): cannot make context current:" << Implementation::eglErrorString(eglGetError());
+    Error() << "Platform::WindowlessEglApplication::makeCurrent(): cannot make context current:" << Implementation::eglErrorString(eglGetError());
+    return false;
+}
+
+bool WindowlessEglContext::release() {
+    if(eglMakeCurrent(_display, EGL_NO_SURFACE, EGL_NO_SURFACE, EGL_NO_CONTEXT))
+        return true;
+
+    Error() << "Platform::WindowlessEglApplication::release(): cannot release current context:" << Implementation::eglErrorString(eglGetError());
     return false;
 }
 

src/Magnum/Platform/WindowlessEglApplication.h

@@ -127,6 +127,14 @@ class WindowlessEglContext {
          */
         bool makeCurrent();
 
+        /**
+         * @brief Release current context
+         *
+         * 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/WindowlessGlxApplication.cpp

@@ -255,6 +255,14 @@ bool WindowlessGlxContext::makeCurrent() {
     return false;
 }
 
+bool WindowlessGlxContext::release() {
+    if(glXMakeContextCurrent(_display, 0, 0, nullptr))
+        return true;
+
+    Error() << "Platform::WindowlessGlxContext::release(): cannot release current context";
+    return false;
+}
+
 WindowlessGlxContext::Configuration::Configuration():
     #ifndef MAGNUM_TARGET_GLES
     _flags{Flag::ForwardCompatible}

src/Magnum/Platform/WindowlessGlxApplication.h

@@ -146,6 +146,14 @@ class WindowlessGlxContext {
          */
         bool makeCurrent();
 
+        /**
+         * @brief Release current context
+         *
+         * 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}