Optionally expose time to screen shaders (#1700)

* Optionally expose time to screen shaders

Since that collides with damage tracking, it will only be done, when
damage tracking is disabled, meaning this comes at no cost for everyone
not using time.

* Rename m_Timer to m_tGlobalTimer

src/render/Shader.hpp

@@ -37,6 +37,8 @@ class CShader {
     GLint  gradientLength;
     GLint  angle;
 
+    GLint  time;
+
     GLint  getUniformLocation(const std::string&);
 
     void   destroy();