vo_opengl: fix compatibility with OpenGL 2.1

The srgb_compand() function passes bvec to mix(), which is apparently
not available on GL 2.1 / GLSL 1.20:

0:0(0): error: no matching function for call to `mix(vec3, vec3, bvec3)'
0:0(0): error: candidates are: float mix(float, float, float)
0:0(0): error:                 vec2 mix(vec2, vec2, vec2)
0:0(0): error:                 vec3 mix(vec3, vec3, vec3)
0:0(0): error:                 vec4 mix(vec4, vec4, vec4)
0:0(0): error:                 vec2 mix(vec2, vec2, float)
0:0(0): error:                 vec3 mix(vec3, vec3, float)
0:0(0): error:                 vec4 mix(vec4, vec4, float)

Also add back disabling color management on older GL, as the
srgb_compand() function is needed for that.

libvo/vo_opengl.c

@@ -1455,6 +1455,10 @@ static void check_gl_features(struct gl_priv *p)
     bool have_fbo = gl->mpgl_caps & MPGL_CAP_FB;
     bool have_srgb = gl->mpgl_caps & MPGL_CAP_SRGB_TEX;
 
+    // srgb_compand() not available
+    if (gl->glsl_version < 130)
+        have_srgb = false;
+
     char *disabled[10];
     int n_disabled = 0;
 
@@ -1488,6 +1492,10 @@ static void check_gl_features(struct gl_priv *p)
         p->use_lut_3d = false;
         disabled[n_disabled++] = "color management (FBO)";
     }
+    if (!have_srgb && p->use_lut_3d) {
+        p->use_lut_3d = false;
+        disabled[n_disabled++] = "color management (sRGB)";
+    }
 
     if (!have_fbo) {
         p->use_scale_sep = false;

libvo/vo_opengl_shaders.glsl

@@ -37,11 +37,14 @@
 # define in varying
 #endif
 
+// Earlier GLSL doesn't support mix() with bvec
+#if __VERSION__ >= 130
 vec3 srgb_compand(vec3 v)
 {
     return mix(1.055 * pow(v, vec3(1.0/2.4)) - vec3(0.055), v * 12.92,
                lessThanEqual(v, vec3(0.0031308)));
 }
+#endif
 
 #!section vertex_all