vulkan_ffv1: always keep the decoder's fltmap descriptor valid

The setup shader statically uses the fltmap binding, but the decoder
only bound a buffer to it for float formats, leaving a dangling
descriptor on everything else, with the write reachable by any
bitstream signalling remap.

Gate remap on a new push-constant flag, so that streams signalling it
without a fltmap buffer error out, and point the descriptor at the
slice feedback buffer when no fltmap exists; the gate guarantees it is
never written through this binding.

libavcodec/ffv1_vulkan.h

@@ -49,6 +49,9 @@ typedef struct FFv1ShaderParams {
     int pic_mode;
     uint32_t slice_size_max;
     uint32_t max_pixels_per_slice;
+
+    /* Decoder-only */
+    uint32_t remap_allowed;
 } FFv1ShaderParams;
 
 #endif /* AVCODEC_FFV1_VULKAN_H */

libavcodec/vulkan/ffv1_common.glsl

@@ -76,6 +76,9 @@ layout (push_constant, scalar) uniform pushConstants {
     int pic_mode;
     uint slice_size_max;
     uint max_pixels_per_slice;
+
+    /* Decoder-only */
+    bool remap_allowed;
 };
 
 #include "rangecoder.glsl"

libavcodec/vulkan/ffv1_dec_setup.comp.glsl

@@ -202,8 +202,15 @@ bool decode_slice_header(uint slice_idx, inout SliceContext sc)
 
         if (micro_version >= 4) {
             sc.remap = get_usymbol(0);
-            if (sc.remap != 0 && decode_remap(slice_idx, sc))
-                return true;
+            if (sc.remap != 0) {
+                /* No fltmap buffer is bound unless the format can remap */
+                if (!remap_allowed) {
+                    sc.remap = 0;
+                    return true;
+                }
+                if (decode_remap(slice_idx, sc))
+                    return true;
+            }
         }
     }
 

libavcodec/vulkan_ffv1.c

@@ -347,13 +347,14 @@ static int vk_ffv1_end_frame(AVCodecContext *avctx)
                                     2*f->slice_count*sizeof(uint32_t),
                                     VK_WHOLE_SIZE,
                                     VK_FORMAT_UNDEFINED);
-    if (fltmap_buf)
-        ff_vk_shader_update_desc_buffer(&ctx->s, exec, &fv->setup,
-                                        1, 3, 0,
-                                        fltmap_buf,
-                                        0,
-                                        VK_WHOLE_SIZE,
-                                        VK_FORMAT_UNDEFINED);
+    /* The binding is statically used by the shader, so it must always hold
+     * a valid buffer. */
+    ff_vk_shader_update_desc_buffer(&ctx->s, exec, &fv->setup,
+                                    1, 3, 0,
+                                    fltmap_buf ? fltmap_buf : slice_feedback,
+                                    0,
+                                    VK_WHOLE_SIZE,
+                                    VK_FORMAT_UNDEFINED);
 
     ff_vk_exec_bind_shader(&ctx->s, exec, &fv->setup);
 
@@ -367,6 +368,7 @@ static int vk_ffv1_end_frame(AVCodecContext *avctx)
         .key_frame = f->picture.f->flags & AV_FRAME_FLAG_KEY,
         .crcref = f->crcref,
         .micro_version = f->micro_version,
+        .remap_allowed = !!fltmap_buf,
     };
 
     for (int i = 0; i < f->quant_table_count; i++) {