Shader: ability to use version directive from shader source.

src/Magnum/Shader.cpp

@@ -558,11 +558,11 @@ Shader::Shader(const Version version, const Type type): _type(type), _id(0) {
         case Version::GLES300: _sources.push_back("#version 300\n"); return;
         #endif
 
-        case Version::None:
-            CORRADE_ASSERT(false, "Shader::Shader(): unsupported version" << version, );
+        /* The user is responsible for (not) adding #version directive */
+        case Version::None: return;
     }
 
-    CORRADE_ASSERT_UNREACHABLE();
+    CORRADE_ASSERT(false, "Shader::Shader(): unsupported version" << version, );
 }
 
 Shader::~Shader() {

src/Magnum/Shader.h

@@ -436,8 +436,10 @@ class MAGNUM_EXPORT Shader: public AbstractObject {
          * @param version   Target version
          * @param type      %Shader type
          *
-         * Creates empty OpenGL shader and adds @c \#version directive at the
-         * beginning.
+         * Creates empty OpenGL shader and adds @c \#version directive
+         * corresponding to @p version parameter at the beginning. If
+         * @ref Version::None is specified, (not) adding the @c \#version
+         * directive is left to the user.
          * @see @fn_gl{CreateShader}
          */
         explicit Shader(Version version, Type type);

src/Magnum/Test/ShaderGLTest.cpp

@@ -39,6 +39,7 @@ class ShaderGLTest: public AbstractOpenGLTester {
         explicit ShaderGLTest();
 
         void construct();
+        void constructNoVersion();
         void constructCopy();
         void constructMove();
 
@@ -51,6 +52,7 @@ class ShaderGLTest: public AbstractOpenGLTester {
 
 ShaderGLTest::ShaderGLTest() {
     addTests({&ShaderGLTest::construct,
+              &ShaderGLTest::constructNoVersion,
               &ShaderGLTest::constructCopy,
               &ShaderGLTest::constructMove,
 
@@ -82,6 +84,11 @@ void ShaderGLTest::construct() {
     MAGNUM_VERIFY_NO_ERROR();
 }
 
+void ShaderGLTest::constructNoVersion() {
+    const Shader shader(Version::None, Shader::Type::Fragment);
+    CORRADE_VERIFY(shader.sources().empty());
+}
+
 void ShaderGLTest::constructCopy() {
     CORRADE_VERIFY(!(std::is_constructible<Shader, const Shader&>{}));
     /* GCC 4.6 doesn't have std::is_assignable */