Fix the case where not having created a GlResource prior to calling Shader::isAvailable() would break context management. (#211)

src/SFML/Graphics/Shader.cpp

@@ -29,6 +29,7 @@
 #include <SFML/Graphics/Shader.hpp>
 #include <SFML/Graphics/Texture.hpp>
 #include <SFML/Graphics/GLCheck.hpp>
+#include <SFML/Window/Context.hpp>
 #include <SFML/System/InputStream.hpp>
 #include <SFML/System/Err.hpp>
 #include <fstream>
@@ -437,15 +438,29 @@ void Shader::bind(const Shader* shader)
 ////////////////////////////////////////////////////////////
 bool Shader::isAvailable()
 {
-    ensureGlContext();
+    static bool available = false;
+    static bool checked = false;
 
-    // Make sure that extensions are initialized
-    priv::ensureExtensionsInit();
+    // Make sure we only have to check once
+    if (!checked)
+    {
+        // Create a temporary context in case the user checks
+        // before a GlResource is created, thus initializing
+        // the shared context
+        Context context;
 
-    return GLEW_ARB_shading_language_100 &&
-           GLEW_ARB_shader_objects       &&
-           GLEW_ARB_vertex_shader        &&
-           GLEW_ARB_fragment_shader;
+        // Make sure that extensions are initialized
+        priv::ensureExtensionsInit();
+
+        available = GLEW_ARB_shading_language_100 &&
+                    GLEW_ARB_shader_objects       &&
+                    GLEW_ARB_vertex_shader        &&
+                    GLEW_ARB_fragment_shader;
+
+        checked = true;
+    }
+
+    return available;
 }