Shaders: don't use explicit attribute location on GL 2.1.

src/Shaders/FlatShader.cpp

@@ -56,7 +56,8 @@ template<std::uint8_t dimensions> FlatShader<dimensions>::FlatShader(): transfor
     attachShader(fragmentShader);
 
     #ifndef MAGNUM_TARGET_GLES
-    if(!Context::current()->isExtensionSupported<Extensions::GL::ARB::explicit_attrib_location>()) {
+    if(!Context::current()->isExtensionSupported<Extensions::GL::ARB::explicit_attrib_location>() ||
+        Context::current()->version() == Version::GL210) {
     #else
     if(!Context::current()->isVersionSupported(Version::GLES300)) {
     #endif

src/Shaders/PhongShader.cpp

@@ -42,7 +42,8 @@ PhongShader::PhongShader(): transformationMatrixUniform(0), projectionMatrixUnif
     attachShader(fragmentShader);
 
     #ifndef MAGNUM_TARGET_GLES
-    if(!Context::current()->isExtensionSupported<Extensions::GL::ARB::explicit_attrib_location>()) {
+    if(!Context::current()->isExtensionSupported<Extensions::GL::ARB::explicit_attrib_location>() ||
+        Context::current()->version() == Version::GL210) {
     #else
     if(!Context::current()->isVersionSupported(Version::GLES300)) {
     #endif

src/Shaders/TextShader.cpp

@@ -57,7 +57,8 @@ template<std::uint8_t dimensions> TextShader<dimensions>::TextShader(): transfor
     AbstractTextShader<dimensions>::attachShader(fragmentShader);
 
     #ifndef MAGNUM_TARGET_GLES
-    if(!Context::current()->isExtensionSupported<Extensions::GL::ARB::explicit_attrib_location>()) {
+    if(!Context::current()->isExtensionSupported<Extensions::GL::ARB::explicit_attrib_location>() ||
+        Context::current()->version() == Version::GL210) {
     #else
     if(!Context::current()->isVersionSupported(Version::GLES300)) {
     #endif

src/Shaders/VertexColorShader.cpp

@@ -48,7 +48,8 @@ template<std::uint8_t dimensions> VertexColorShader<dimensions>::VertexColorShad
     attachShader(fragmentShader);
 
     #ifndef MAGNUM_TARGET_GLES
-    if(!Context::current()->isExtensionSupported<Extensions::GL::ARB::explicit_attrib_location>()) {
+    if(!Context::current()->isExtensionSupported<Extensions::GL::ARB::explicit_attrib_location>() ||
+        Context::current()->version() == Version::GL210) {
     #else
     if(!Context::current()->isVersionSupported(Version::GLES300)) {
     #endif