vo_gpu: improve numerical accuracy of PQ OETF constant

Not a huge deal, but we can do the division in C, which makes the float
constant larger.

video/out/gpu/video_shaders.c

@@ -380,7 +380,7 @@ void pass_linearize(struct gl_shader_cache *sc, enum mp_csp_trc trc)
         GLSLF("color.rgb = max(color.rgb - vec3(%f), vec3(0.0)) \n"
               "             / (vec3(%f) - vec3(%f) * color.rgb);\n",
               PQ_C1, PQ_C2, PQ_C3);
-        GLSLF("color.rgb = pow(color.rgb, vec3(1.0/%f));\n", PQ_M1);
+        GLSLF("color.rgb = pow(color.rgb, vec3(%f));\n", 1.0 / PQ_M1);
         // PQ's output range is 0-10000, but we need it to be relative to to
         // MP_REF_WHITE instead, so rescale
         GLSLF("color.rgb *= vec3(%f);\n", 10000 / MP_REF_WHITE);