OpenGL2: Don't mix drawing to default framebuffer and FBO

Don't draw the world scene to a separate FBO from the rest of the
screen.

This fixes the world scene having HOM instead of seeing through to the
previously drawn content. World of Padman uses this to have a separate
3D scene for the sky and world in wop_padship for dynamic skybox.

This also makes r_ext_framebuffer_multisample apply to HUD models
instead of depending on r_ext_multisample (which doesn't work on Linux
with some Intel graphics).

code/renderergl2/tr_postprocess.h

@@ -28,6 +28,6 @@ Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 void RB_ToneMap(FBO_t *hdrFbo, ivec4_t hdrBox, FBO_t *ldrFbo, ivec4_t ldrBox, int autoExposure);
 void RB_BokehBlur(FBO_t *src, ivec4_t srcBox, FBO_t *dst, ivec4_t dstBox, float blur);
 void RB_SunRays(FBO_t *srcFbo, ivec4_t srcBox, FBO_t *dstFbo, ivec4_t dstBox);
-void RB_GaussianBlur(float blur);
+void RB_GaussianBlur(FBO_t *srcFbo, FBO_t *dstFbo, float blur);
 
 #endif