Refactor GLSL shader compilation

-Used a more consistent set of keywords for the shader
-Remove all harcoded entry points
-Re-wrote the GLSL shader parser, new system is more flexible. Allows any entry point organization.
-Entry point for sky shaders is now sky().
-Entry point for particle shaders is now process().

servers/rendering/renderer_rd/shaders/cubemap_downsampler.glsl

@@ -22,7 +22,7 @@
 
 #version 450
 
-VERSION_DEFINES
+#VERSION_DEFINES
 
 #define BLOCK_SIZE 8