Change naming convention, merge texture/shaders

Reduce the number of files in project by merging texture and shaders into
assets/renderer file. It's better to keep code local to where it is rather than
splitting them into "self-contained modules". It becomes easier to understand
reading top-to-bottom and there's is less jumping around which can increase the
difficulty of comprehending the code.

Convert names to use more of a <namespace>_<object><Action> format.

src/dengine.c

@@ -1,10 +1,10 @@
 #include "Dengine/AssetManager.h"
+#include "Dengine/Asteroid.h"
 #include "Dengine/Common.h"
 #include "Dengine/Debug.h"
 #include "Dengine/Math.h"
-#include "Dengine/OpenGL.h"
 #include "Dengine/MemoryArena.h"
-#include "Dengine/Asteroid.h"
+#include "Dengine/OpenGL.h"
 
 INTERNAL inline void processKey(KeyState *state, int action)
 {