GL: opt-in bloom ([video] bloom), sourced from 3D and additive primitives only

[video] bloom = 0.0 (off, default) .. 2.0 adds a glow around lights on the
OpenGL backend. PSX_BLOOM overrides it for one run; {"cmd":"bloom"} reads or
changes strength, threshold and a debug view live. It is a plain [video] key,
so [quality.*] presets can switch it off on low-end machines.

The HUD never glows: with bloom on every textured and untextured draw is
followed by two stencil-only passes of the same vertices that keep a
"may glow" mark in stencil bit 1 (bit 0 stays the mask-bit mirror). Opaque
primitives clear it under their coverage, then PGXP-proven 3D primitives and
additive semi-transparent texels/primitives (modes 1 and 3) set it, painter
ordered like the colour; 3D and non-3D primitives never share a batch while
bloom is on. Semi-transparent primitives leave the mark alone and the PGXP
depth-clear pass (colour writes off) marks nothing. Without PGXP geometry only
additive primitives glow.

The pass runs on the composed game image before AA and the OSD: the presented
surface's marks are blitted into a quarter-resolution stencil, a soft-knee
bright pass runs under it, three separable Gaussian iterations spread it and a
screen blend adds it back. The marks follow the picture: native-wide centre
copies and Smooth motion surfaces (redraw and reprojection) copy stencil with
colour, and generated frames get the same pass. Off: nothing runs, no state
changes, and the composed frame is byte-identical to master.

test_gl_bloom (gl_bloom_test): off is byte-identical before/after bloom and
with --dump against master; on, a bright 2D panel and a panel over a lit 3D
polygon are byte-identical to off, an additive and a 3D polygon glow; guest
VRAM is unchanged; on this thread and on the render thread.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

runtime/src/main.cpp

@@ -1229,6 +1229,7 @@ extern "C" EMSCRIPTEN_KEEPALIVE void psx_web_set_smooth_60fps(int enabled) {
 static int           g_video_scale = 1;     /* internal-resolution SSAA factor */
 static bool          g_video_aa    = true;  /* linear present filtering */
 static int           g_video_post_aa = 0;   /* [video] antialiasing_mode / PSX_AA_MODE (GL_POST_AA_*) */
+static float         g_video_bloom = 0.0f;  /* [video] bloom (PSX_BLOOM overrides) */
 static int           g_video_ss_milli = 1000; /* [video] supersample / PSX_SUPERSAMPLE */
 /* FMV present reconstruction (VIDEO_FMV_FILTER_*), pushed to the GL renderer
  * once the config is resolved. Only consulted while g_video_aa is on. */
@@ -16332,6 +16333,7 @@ int main(int argc, char** argv) {
             }
             g_video_aa         = gc.runtime.video_antialiasing;
             g_video_post_aa    = gc.runtime.video_antialiasing_mode;
+            g_video_bloom      = gc.runtime.video_bloom;
             g_video_ss_milli   = gc.runtime.video_supersample_milli;
             g_video_texfilter  = gc.runtime.video_texture_filter;
             g_video_fmv_filter = gc.runtime.video_fmv_filter;
@@ -19114,6 +19116,7 @@ session_reboot:
         }
         gl_renderer_set_swap_interval(0);
         (void)gl_renderer_set_post_aa(g_video_post_aa);
+        gl_renderer_set_bloom(g_video_bloom);
         g_gl_active = gl_renderer_init_context(s_headless_gl_window) != 0;
         if (!g_gl_active || gr_backend() != GR_BACKEND_OPENGL ||
             !(SDL_GetWindowFlags(s_headless_gl_window) & SDL_WINDOW_HIDDEN)) {
@@ -19326,6 +19329,7 @@ session_reboot:
         gl_renderer_set_dynamic_resolution(
             ((dynres_requested() || dbg_live_scale) && g_video_scale_applies) ? 1 : 0);
         (void)gl_renderer_set_post_aa(g_video_post_aa);
+        gl_renderer_set_bloom(g_video_bloom);
         g_gl_active = (gl_renderer_init_context(sdl_window) != 0);
         dynres_setup();
         if (dbg_live_scale && g_gl_active) {