Change all OpenGL extension constants to be const. Fix OpenGL ES and mobile.

* OpenGL ES wasn't compiling because there's no `GL_R16`, `GL_RG16`, `GL_RGB16`,
  `GL_RGBA16`. Replaced by their `_EXT` versions, but it wasn't compiling
  because they're defined as `var` instead of `const` and they were being used
  in a `func`.
* The GL API files were changed to fix that.
* After loading the scene, mobile platforms were throwing an error because the
  renderer was not initialized yet. That's been fixed.
* Remove unsed `gl-basic.nim`.

src/graphics/texture.nim

@@ -156,10 +156,10 @@ func toInternalFormat*(format: TextureFormat): GLenum =
         of RG_u8: GL_RG8
         of RGB_u8: GL_RGB8
         of RGBA_u8: GL_RGBA8
-        of R_u16: GL_R16
+        of R_u16: GL_R16_EXT
-        of RG_u16: GL_RG16
+        of RG_u16: GL_RG16_EXT
-        of RGB_u16: GL_RGB16
+        of RGB_u16: GL_RGB16_EXT
-        of RGBA_u16: GL_RGBA16
+        of RGBA_u16: GL_RGBA16_EXT
         of R_f16: GL_R16F
         of RG_f16: GL_RG16F
         of RGB_f16: GL_RGB16F
@@ -567,7 +567,6 @@ proc getDimensionsFormat(p: pointer, len: int): (int, int, TextureFormat) =
             const toHDR = when myouConvertHdrToFloat16: (R_f16.int-R_u8.int) else: (R_f32.int-R_u8.int)
             let format = (R_u8.int - 1 + channels +
                 hdr.int * toHDR + is16.int * (R_u16.int-R_u8.int)).TextureFormat
-            dump format
             return (width, height, format)
 
 func to_sRGB*(format: TextureFormat): TextureFormat =

src/loaders/blend.nim

@@ -593,7 +593,8 @@ method loadScene*(self: BlendLoader, name: string="", scene: Scene=nil, callback
                 # finished
                 if was_first_scene and not scene.enabled:
                     echo "Warning: Your scene is not enabled, use 'scene.enable_render()' or 'scene.enable_all()'"
-                scene.render_all_cubemaps(true)
+                self.engine.renderer.enqueue proc() =
+                    scene.render_all_cubemaps(true)
                 return
         assert false, &"Scene {name} not found"