[shanyuduo]

Problem

Godot 4.7's RenderingDevice now strictly validates the push constant size against what the compute shader declares. yuv_to_rgb.glsl declares a 4-byte push constant (bool use_alpha), but FFmpegVideoStreamPlayback::draw() uploads the full 16-byte padded struct (PushConstant { uint32_t use_alpha; uint8_t padding[11]; }).

compute_list_set_push_constant() is therefore rejected on every frame:

ERROR: This compute pipeline requires (4) bytes of push constant data, supplied: (16)
ERROR: The shader in this pipeline requires a push constant to be set before drawing, but it's not present.

and the pipeline never executes, so decoded YUV frames are never converted: video shows nothing under the Forward+/Mobile renderers on Godot 4.7+ (Vulkan logs the error every frame; D3D12 fails silently).

This is the same issue reported in #76 — @EIREXE identified the breaking RD API change there, but the fix hasn't landed yet, so this PR proposes it.

Fix

Pass only the 4 bytes the shader actually declares, skipping the struct padding:

 	PackedByteArray push_constant_data;
-	push_constant_data.resize(sizeof(push_constant));
-	memcpy(push_constant_data.ptrw(), &push_constant, push_constant_data.size());
+	// Godot 4.7's RenderingDevice strictly validates the push constant size
+	// against what the compute shader declares (4 bytes here: bool use_alpha).
+	// Uploading the padded 16-byte struct gets the call rejected on every
+	// frame, so the YUV->RGB pipeline never executes and video never renders
+	// in Forward+/Mobile on Godot 4.7+. Pass only the declared 4 bytes.
+	push_constant_data.resize(4);
+	memcpy(push_constant_data.ptrw(), &push_constant, 4);

This stays correct on older Godot versions, where the check only requires the upload to be at least the declared size.

Verification

• Godot 4.7.2 stable, Forward+, both Vulkan and D3D12, Intel iGPU (RaptorLake-S):
  a 1440p@120 H.264 video renders correctly and the per-frame push constant errors are
  gone (0 errors, previously 2 errors per rendered frame).
• Built via gdextension_build SCons pipeline (mingw gcc 16.2, ffmpeg_path pointing
  at the godot FFmpeg build);
  both template_debug and template_release dlls verified in a real project.

[spaceyspacestar]

This fix works great.
Using: Godot 4.8.dev4.mono using Forward+, tested on both d3d12 and Vulkan

[EIREXE]

My apolocheese for the delay, I will review it