vo_opengl: also expose NAME_mul for user shaders

This is exposed so that bjin/mpv-prescalers can use textureGatherOffset
for performance.

Since there are now quite a lot of parameters where it isn't quite clear
why they're all defined, add a paragraph to the man page that explains
them a bit.

DOCS/man/options.rst

@@ -4274,10 +4274,18 @@ The following video options are currently all specific to ``--vo=opengl`` and
         to texture coordinates)
     vec2 NAME_pt
         The (unrotated) size of a single pixel, range [0,1].
+    float NAME_mul
+        The coefficient that needs to be multiplied into the texture contents
+        in order to normalize it to the range [0,1].
     sampler NAME_raw
         The raw bound texture itself. The use of this should be avoided unless
         absolutely necessary.
 
+    Normally, users should use either NAME_tex or NAME_texOff to read from the
+    texture. For some shaders however , it can be better for performance to do
+    custom sampling from NAME_raw, in which case care needs to be taken to
+    respect NAME_mul and NAME_rot.
+
     In addition to these parameters, the following uniforms are also globally
     available:
 

video/out/opengl/video.c

@@ -1171,10 +1171,11 @@ static void hook_prelude(struct gl_video *p, const char *name, int id,
     GLSLHF("#define %s_size texture_size%d\n", name, id);
     GLSLHF("#define %s_rot texture_rot%d\n", name, id);
     GLSLHF("#define %s_pt pixel_size%d\n", name, id);
+    GLSLHF("#define %s_mul %f\n", name, tex.multiplier);
 
     // Set up the sampling functions
-    GLSLHF("#define %s_tex(pos) (%f * vec4(texture(%s_raw, pos)).%s)\n",
-           name, tex.multiplier, name, get_tex_swizzle(&tex));
+    GLSLHF("#define %s_tex(pos) (%s_mul * vec4(texture(%s_raw, pos)).%s)\n",
+           name, name, name, get_tex_swizzle(&tex));
 
     // Since the extra matrix multiplication impacts performance,
     // skip it unless the texture was actually rotated