tr_backend: fix FXAA sampling by using GL_LINEAR on currentRender

- fix FXAA by using GL_LINEAR on currentRender,
- restore GL_NEAREST after that to not break other effects.

Author: illwieckz

src/engine/renderer/tr_backend.cpp

@@ -1625,11 +1625,19 @@ void RB_FXAA()
 		GL_BindToTMU( 0, tr.currentRenderImage[backEnd.currentMainFBO] )
 	);
 
+	// The framebuffer should use GL_LINEAR for FXAA to work.
+	glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR );
+	glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );
+
 	// This shader is run last, so let it render to screen.
 	R_BindNullFBO();
 
 	Tess_InstantScreenSpaceQuad();
 
+	// Restore GL_NEAREST to not break other effects.
+	glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST );
+	glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST );
+
 	GL_CheckErrors();
 }