Converted rlgl module in header-only

This change allows rlgl.h usage as independent single-file header-only module... still some tweaks required, like removing GLAD dependency... required extensions could be manually loaded!

Also removed shader_distortion.h, embedded in rlgl.h

examples/core/core_vr_simulator.c

@@ -31,6 +31,7 @@ int main()
     camera.target = (Vector3){ 0.0f, 2.0f, 0.0f };      // Camera looking at point
     camera.up = (Vector3){ 0.0f, 1.0f, 0.0f };          // Camera up vector (rotation towards target)
     camera.fovy = 60.0f;                                // Camera field-of-view Y
+    camera.type = CAMERA_PERSPECTIVE;                   // Camera type
     
     Vector3 cubePosition = { 0.0f, 0.0f, 0.0f };