shaders/colorspace: use perceptually linear pure BPC

It's not that hard to convert to PQ and back in pure GLSL, so we might
as well do that for pure BPC. Produces significantly better results (no
clipping) than naive/linear black point stretching.

src/shaders/colorspace.c

@@ -1366,10 +1366,38 @@ static void tone_map(pl_shader sh,
 
     } else if (pure_bpc) {
 
-        // Pure black point compensation
-        const float scale = (dst_max - dst_min) / (src_max - src_min);
-        GLSL("#define tone_map(x) (%s * (x) + %s) \n",
-             SH_FLOAT(scale), SH_FLOAT(dst_min - scale * src_min));
+        const float pq_src_min = pl_hdr_rescale(PL_HDR_NORM, PL_HDR_PQ, src_min);
+        const float pq_src_max = pl_hdr_rescale(PL_HDR_NORM, PL_HDR_PQ, src_max);
+        const float pq_dst_min = pl_hdr_rescale(PL_HDR_NORM, PL_HDR_PQ, dst_min);
+
+        ident_t bpc = sh_fresh(sh, "bpc_pq");
+        GLSLH("float %s(float x) {                          \n"
+             // PQ OETF
+             "    x *= %f;                                  \n"
+             "    x = pow(max(x, 0.0), %f);                 \n"
+             "    x = (%f + %f * x) / (1.0 + %f * x);       \n"
+             "    x = pow(x, %f);                           \n"
+             // Stretch the black point
+             "    x -= %s;                                  \n"
+             "    x *= %s;                                  \n"
+             "    x += %s;                                  \n"
+             // PQ EOTF
+             "    x = pow(max(x, 0.0), 1.0/%f);             \n"
+             "    x = max(x - %f, 0.0) / (%f - %f * x);     \n"
+             "    x = pow(x, 1.0 / %f);                     \n"
+             "    x *= %f;                                  \n"
+             "    return x;                                 \n"
+             "}                                             \n",
+             bpc,
+             PL_COLOR_SDR_WHITE / 10000.0,
+             PQ_M1, PQ_C1, PQ_C2, PQ_C3, PQ_M2,
+             SH_FLOAT(pq_src_min),
+             SH_FLOAT((pq_src_max - pq_dst_min) / (pq_src_max - pq_src_min)),
+             SH_FLOAT(pq_dst_min),
+             PQ_M2, PQ_C1, PQ_C2, PQ_C3, PQ_M1,
+             10000.0 / PL_COLOR_SDR_WHITE);
+
+        GLSL("#define tone_map(x) (%s(x)) \n", bpc);
 
     } else if (lut && dynamic_peak) {