UPBGE: Allow creating custom filter without shader source.

Previously the user was obligated to specify a fragment source
when calling filter.addFilter for custom filter. But if the user
want to use a custom shader it was impossible to not failed the first
compilation and then call setSourceList.
To avoid an useless compilation failure, the function addFilter allow
creating a custom shader without receiving any fragment source.
This produces a empty filter unused in rendering while the user doesn't
called setSourceList.

doc/python_api/rst/bge_types/bge.types.KX_2DFilterManager.rst

@@ -30,7 +30,7 @@ base class --- :class:`PyObjectPlus`
 
       :type type: integer
       :arg fragmentProgram: The filter shader fragment program.
-          Specified only if :data:`type` is :data:`bge.logic.RAS_2DFILTER_CUSTOMFILTER`. (optional)
+          Used only if :data:`type` is :data:`bge.logic.RAS_2DFILTER_CUSTOMFILTER`, if empty or not specified the filter is created without shader, waiting call to :data:`BL_Shader.setSourceList`. (optional)
       :type fragmentProgram: string
       :return: The 2D Filter.
       :rtype: :class:`KX_2DFilter`

source/gameengine/Rasterizer/RAS_2DFilter.cpp

@@ -58,10 +58,12 @@ RAS_2DFilter::RAS_2DFilter(RAS_2DFilterData& data)
 		m_predefinedUniforms[i] = -1;
 	}
 
-	m_progs[VERTEX_PROGRAM] = std::string(datatoc_RAS_VertexShader2DFilter_glsl);
-	m_progs[FRAGMENT_PROGRAM] = data.shaderText;
+	if (!data.shaderText.empty()) {
+		m_progs[VERTEX_PROGRAM] = std::string(datatoc_RAS_VertexShader2DFilter_glsl);
+		m_progs[FRAGMENT_PROGRAM] = data.shaderText;
 
-	LinkProgram();
+		LinkProgram();
+	}
 }
 
 RAS_2DFilter::~RAS_2DFilter()