Add OpenGLContext::UseNoProgram()

Will be useful in cases where there is some external library code that uses
shaders outside of our OpenGLContext.

Change-Id: I59c57e3225f55d13e69b6a9b7c0db1a7487c586d

include/vcl/opengl/OpenGLContext.hxx

@@ -167,6 +167,7 @@ public:
     // retrieve a program from the cache or compile/link it
     OpenGLProgram*      GetProgram( const OUString& rVertexShader, const OUString& rFragmentShader, const OString& preamble = "" );
     OpenGLProgram*      UseProgram( const OUString& rVertexShader, const OUString& rFragmentShader, const OString& preamble = "" );
+    void                UseNoProgram();
 
     /// Is this GL context the current context ?
     bool isCurrent();

vcl/source/opengl/OpenGLContext.cxx

@@ -1732,4 +1732,14 @@ OpenGLProgram* OpenGLContext::UseProgram( const OUString& rVertexShader, const O
     return mpCurrentProgram;
 }
 
+void OpenGLContext::UseNoProgram()
+{
+    if( mpCurrentProgram == NULL )
+        return;
+
+    mpCurrentProgram = NULL;
+    glUseProgram( 0 );
+    CHECK_GL_ERROR();
+}
+
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