refactor GLSLExtension.cpp & add 'param' shader preprocessor directive & add PostEffect class (WIP)

test/coders/GLSLExtension.cpp

@@ -0,0 +1,32 @@
+#include <gtest/gtest.h>
+
+#include "coders/commons.hpp"
+#include "coders/GLSLExtension.hpp"
+
+TEST(GLSLExtension, processing) {
+    GLSLExtension glsl;
+    glsl.addHeader("sum", 
+        "// sum function for glsl\n"
+        "float sum(float a, float b) {\n"
+        "    return a + b;\n"
+        "}\n"
+    );
+    try {
+        auto processed = glsl.process("test.glsl",
+            "in vec2 v_uv;\n"
+            "uniform sampler2D u_screen;\n"
+            "\n"
+            "#include /* hell\no */  <   sum     >\n"
+            "#param float p_intensity\n"
+            "\n"
+            "vec4 effect() {\n"
+            "    vec4 color = texture(u_screen, v_uv);\n"
+            "    return mix(color, 1.0 - color, p_intensity);\n"
+            "}\n",
+        false);
+        std::cout << processed << std::endl;
+    } catch (const parsing_error& err) {
+        std::cerr << err.errorLog() << std::endl;
+        throw;
+    }
+}