GPU: Remove GPU_INDEX_U8

This type of indices is not natively supported on modern GPU and
gives warning on some implementation. The memory savings it
provides is also quite minimal and unlikely to be visible on
nowadays hardware.

This remove some uneeded struct members and makes primitive
restart always enabled by default. This can be broken by addons
if they are not careful enough but many other states have this
problem.

Also leverage GL_PRIMITIVE_RESTART_FIXED_INDEX if
ARB_ES3_compatibility is supported. This removes all API calls
to change restart index depending on indices length.

source/blender/gpu/intern/gpu_draw.c

@@ -1372,6 +1372,16 @@ void GPU_state_init(void)
   glDisable(GL_CULL_FACE);
 
   gpu_disable_multisample();
+
+  /* This is a bit dangerous since addons could change this. */
+  glEnable(GL_PRIMITIVE_RESTART);
+  glPrimitiveRestartIndex((GLuint)0xFFFFFFFF);
+
+  /* TODO: Should become default. But needs at least GL 4.3 */
+  if (GLEW_ARB_ES3_compatibility) {
+    /* Takes predecence over GL_PRIMITIVE_RESTART */
+    glEnable(GL_PRIMITIVE_RESTART_FIXED_INDEX);
+  }
 }
 
 /** \name Framebuffer color depth, for selection codes