Properly detect effects which need OpenGL 2

Effects that need GLSL require that OpenGL 2 is active.
It does not matter whether the GPU would support it if
OpenGL 2 is not used.

Cherry-picked from db42152d

CCBUG: 299426

kwin/effects/explosion/explosion.cpp

@@ -60,8 +60,11 @@ ExplosionEffect::~ExplosionEffect()
 
 bool ExplosionEffect::supported()
 {
-    return GLPlatform::instance()->supports(GLSL) &&
-           (effects->compositingType() == OpenGLCompositing);
+    if (effects->compositingType() == OpenGLCompositing) {
+        return ShaderManager::instance()->isValid();
+    } else {
+        return false;
+    }
 }
 
 bool ExplosionEffect::loadData()

kwin/effects/invert/invert.cpp

@@ -64,8 +64,11 @@ InvertEffect::~InvertEffect()
 
 bool InvertEffect::supported()
 {
-    return GLPlatform::instance()->supports(GLSL) &&
-           (effects->compositingType() == OpenGLCompositing);
+    if (effects->compositingType() == OpenGLCompositing) {
+        return ShaderManager::instance()->isValid();
+    } else {
+        return false;
+    }
 }
 
 bool InvertEffect::loadData()

kwin/effects/lookingglass/lookingglass.cpp

@@ -79,9 +79,11 @@ LookingGlassEffect::~LookingGlassEffect()
 
 bool LookingGlassEffect::supported()
 {
-    return GLRenderTarget::supported() &&
-           GLPlatform::instance()->supports(GLSL) &&
-           (effects->compositingType() == OpenGLCompositing);
+    if (effects->compositingType() == OpenGLCompositing) {
+        return ShaderManager::instance()->isValid() && GLRenderTarget::supported();
+    } else {
+        return false;
+    }
 }
 
 void LookingGlassEffect::reconfigure(ReconfigureFlags)