gl_video: explicitly clamp colormatrix output

This could lead to quite visible artifacts when using an appropriate ICC
and float FBOs. The float FBOs allow storing out of range values, and my
guess is that the rest of the precessing chain elevated these out of
range values, resulting in artifacts.

video/out/gl_video_shaders.glsl

@@ -358,6 +358,7 @@ void main() {
 #endif
 #ifdef USE_COLORMATRIX
     color = mat3(colormatrix) * color + colormatrix[3];
+    color = clamp(color, 0, 1);
 #endif
 #ifdef USE_CONV_GAMMA
     color = pow(color, vec3(conv_gamma));