OpenGL: prepare GLSL for version 3.3

- use in/out instead of attribute/varying
- use named output instead of gl_FragColor
- use texture() instead of the multitude of older texture sampling functions

The #if __VERSION__ == 120 paths (needed on Mac) will be removed after we switch to 3.3 core profile.

Part of T49165 (general OpenGL upgrade)

source/blender/gpu/shaders/gpu_shader_vertex_world.glsl

@@ -1,7 +1,11 @@
 
-varying vec3 varposition;
-varying vec3 varnormal;
-
+#if __VERSION__ == 120
+  varying vec3 varposition;
+  varying vec3 varnormal;
+#else
+  out vec3 varposition;
+  out vec3 varnormal;
+#endif
 
 /* Color, keep in sync with: gpu_shader_vertex.glsl */