[kwineffects] Add a generateCustomShader to ShaderManager

This method allows to generate a shader with ShaderTraits but one
part (e.g. vertex and/or fragment) to be replaced by custom shader
code.

Thus it can be used as a replacement for the load*(ShaderType) variants
and allows more effects to transition to shaders using traits.

libkwineffects/kwinglutils.cpp

@@ -987,8 +987,13 @@ QByteArray ShaderManager::generateFragmentSource(ShaderTraits traits) const
 
 GLShader *ShaderManager::generateShader(ShaderTraits traits)
 {
-    const QByteArray vertex   = generateVertexSource(traits);
-    const QByteArray fragment = generateFragmentSource(traits);
+    return generateCustomShader(traits);
+}
+
+GLShader *ShaderManager::generateCustomShader(ShaderTraits traits, const QByteArray &vertexSource, const QByteArray &fragmentSource)
+{
+    const QByteArray vertex   = vertexSource.isEmpty() ? generateVertexSource(traits) : vertexSource;
+    const QByteArray fragment = fragmentSource.isEmpty() ? generateFragmentSource(traits) : fragmentSource;
 
 #if 0
     qCDebug(LIBKWINGLUTILS) << "**************";

libkwineffects/kwinglutils.h

@@ -361,6 +361,24 @@ public:
      **/
     GLShader *loadShaderFromCode(const QByteArray &vertexSource, const QByteArray &fragmentSource);
 
+    /**
+     * Creates a custom shader with the given @p traits and custom @p vertexSource and or @p fragmentSource.
+     * If the @p vertexSource is empty a vertex shader with the given @p traits is generated.
+     * If it is not empty the @p vertexSource is used as the source for the vertex shader.
+     *
+     * The same applies for argument @p fragmentSource just for the fragment shader.
+     *
+     * So if both @p vertesSource and @p fragmentSource are provided the @p traits are ignored.
+     * If neither are provided a new shader following the @p traits is generated.
+     *
+     * @param traits The shader traits for generating the shader
+     * @param vertesSource optional vertex shader source code to be used instead of shader traits
+     * @param fragmentSource optional fragment shader source code to be used instead of shader traits
+     * @return new generated shader
+     * @since 5.6
+     **/
+    GLShader *generateCustomShader(ShaderTraits traits, const QByteArray &vertexSource = QByteArray(), const QByteArray &fragmentSource = QByteArray());
+
     /**
      * Compiles and tests the dynamically generated shaders.
      * Returns true if successful and false otherwise.