Fixed incorrect management of GL textures used with the sky sphere

When resetting GL, any acquired textures should either be deleted prior
to setting up a new render context and clear the variables used to track
their identifiers on application side at this time, or, simply "forget"
about the uploaded content and clear the identifier tracking values.

doomsday/engine/portable/include/r_sky.h

@@ -113,7 +113,7 @@ void R_SetupSky(ded_sky_t* sky);
 /**
  * Mark all skies as requiring a full update. Called during engine/renderer reset.
  */
-void R_SkyUpdate(void);
+void R_ReleaseGLTexturesForSkies(void);
 
 void R_SetupSkyModels(ded_sky_t* sky);
 void R_SetupSkySphereParamsForSkyLayer(rendskysphereparams_t* params, int layer);

doomsday/engine/portable/src/gl_texmanager.c

@@ -1660,6 +1660,7 @@ void GL_ReleaseRuntimeTextures(void)
     GL_ReleaseTexturesForRawImages();
 
     R_ReleaseGLTexturesForSkins();
+    R_ReleaseGLTexturesForSkies();
     Rend_ParticleReleaseExtraTextures();
     Fonts_ReleaseRuntimeGLTextures();
 }
@@ -3256,7 +3257,6 @@ static int doTexReset(void* parm)
     /// \todo re-upload ALL textures currently in use.
     GL_LoadSystemTextures();
     Rend_ParticleLoadExtraTextures();
-    R_SkyUpdate();
 
     GL_PruneTextureVariantSpecifications();
 

doomsday/engine/portable/src/r_main.c

@@ -477,7 +477,6 @@ void R_Update(void)
 
     Def_PostInit();
     P_UpdateParticleGens(); // Defs might've changed.
-    R_SkyUpdate();
 
     // Reset the archived map cache (the available maps may have changed).
     DAM_Init();

doomsday/engine/portable/src/r_sky.c

@@ -388,7 +388,7 @@ void R_SkyTicker(void)
     }
 }
 
-void R_SkyUpdate(void)
+void R_ReleaseGLTexturesForSkies(void)
 {
     if(novideo || isDedicated)
         return;