gfx/drivers_shader/shader_gl_cg: mask IN.frame_count when no modulo declared

The Cg shader 'IN.frame_count' input is bound via cg_gl_set_param_1f,
which forces a (float) cast of an unsigned monotonic counter. fp32
integer precision dies at 2^24 = 16,777,216, so at 60 fps any Cg
shader that uses IN.frame_count for time-based animation freezes its
counter after roughly 77 hours of continuous play.

The frame_count_mod field on each shader pass is the documented
defense, but it is opt-in and most presets do not set it. Add a
default 24-bit mask when no modulo is declared, so the cast stays
bit-exact in perpetuity and existing Cg shaders that key animation
off IN.frame_count keep working in long sessions.

Presets that do declare frame_count_mod are unaffected.

gfx/drivers_shader/shader_gl_cg.c

@@ -455,6 +455,13 @@ static void gl_cg_set_params(void *dat, void *shader_data)
       unsigned modulo = cg->shader->pass[cg->active_idx - 1].frame_count_mod;
       if (modulo)
          frame_count %= modulo;
+      else
+         /* fp32 mantissa is 23 bits; integers above 2^24 cannot be
+          * represented exactly. Mask to 24 bits when the shader pass
+          * has not declared its own modulo, so (float)frame_count stays
+          * bit-exact and time-based Cg shaders keep animating beyond
+          * ~77 h of continuous play. */
+         frame_count &= 0xFFFFFFu;
 
       cg_gl_set_param_1f(cg->prg[cg->active_idx].frame_cnt_f, (float)frame_count);
       cg_gl_set_param_1f(cg->prg[cg->active_idx].frame_cnt_v, (float)frame_count);