Fix screenshots with read-only depth buffer

When the read-only depth buffer is used (if r_readonlyDepthBuffer is 2,
or if it's 1 but the driver lacks certain extensions), the screenshots
were done from the 3d rendering buffer instead of the final framebuffer.
This caused screenshots to lack tonemapping, color grading and 2D
rendering. Fix it by resetting GL_READ_FRAMEBUFFER to the default value
after it is used.

Author: slipher

src/engine/renderer/tr_backend.cpp

@@ -2712,7 +2712,8 @@ static void RB_RenderView( bool depthPass )
 		int x0 = backEnd.viewParms.viewportX, x1 = x0 + backEnd.viewParms.viewportWidth;
 		int y0 = backEnd.viewParms.viewportY, y1 = y0 + backEnd.viewParms.viewportHeight;
 		GL_fboShim.glBlitFramebuffer( x0, y0, x1, y1, x0, y0, x1, y1, GL_DEPTH_BUFFER_BIT, GL_NEAREST );
-		R_BindFBO( currentDrawFBO );
+		R_BindFBO( GL_READ_FRAMEBUFFER, nullptr );
+		R_BindFBO( GL_DRAW_FRAMEBUFFER, currentDrawFBO );
 	}
 
 	if( tr.refdef.blurVec[0] != 0.0f ||