Add MPV render context API for OpenGL rendering

Created render.go with CGO bindings for mpv_render_context:
- NewRenderContext: Create render context with parameters
- SetUpdateCallback: Register frame update callbacks
- Render: Issue render calls with FBO/dimension params
- Fixed CGO type conversions (int vs mpv_render_param_type)

This enables hardware-accelerated OpenGL rendering of MPV video
frames in the GTK player.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

player/mpvembed/render.go

@@ -0,0 +1,82 @@
+package mpvembed
+
+/*
+#cgo pkg-config: mpv
+#include <mpv/render.h>
+#include <stdlib.h>
+
+static inline const char* mpv_errstr_render(int err) { return mpv_error_string(err); }
+*/
+import "C"
+import (
+	"fmt"
+	"unsafe"
+)
+
+// RenderParam is a small helper to build mpv_render_param arrays.
+type RenderParam struct {
+	Type int
+	Data unsafe.Pointer
+}
+
+// RenderContext wraps mpv_render_context for render API (OpenGL/Vulkan, etc.).
+type RenderContext struct {
+	ctx *C.mpv_render_context
+}
+
+// NewRenderContext creates a render context for the given client with the provided params.
+// The params slice is terminated with MPV_RENDER_PARAM_INVALID automatically.
+func NewRenderContext(c *Client, params []RenderParam) (*RenderContext, error) {
+	if c == nil || c.handle == nil {
+		return nil, fmt.Errorf("mpv client is nil")
+	}
+	cparams := make([]C.mpv_render_param, len(params)+1)
+	for i, p := range params {
+		cparams[i]._type = C.int(p.Type)
+		cparams[i].data = p.Data
+	}
+	cparams[len(params)]._type = C.MPV_RENDER_PARAM_INVALID
+
+	var rctx *C.mpv_render_context
+	if res := C.mpv_render_context_create(&rctx, c.handle, &cparams[0]); res < 0 {
+		return nil, fmt.Errorf("mpv_render_context_create failed: %s", C.GoString(C.mpv_errstr_render(res)))
+	}
+	return &RenderContext{ctx: rctx}, nil
+}
+
+// Destroy frees the render context.
+func (r *RenderContext) Destroy() {
+	if r != nil && r.ctx != nil {
+		C.mpv_render_context_free(r.ctx)
+		r.ctx = nil
+	}
+}
+
+// SetUpdateCallback registers a callback that mpv will invoke when a new frame should be drawn.
+// The callback must be thread-safe.
+func (r *RenderContext) SetUpdateCallback(cb unsafe.Pointer, userdata unsafe.Pointer) error {
+	if r == nil || r.ctx == nil {
+		return fmt.Errorf("render context is nil")
+	}
+	C.mpv_render_context_set_update_callback(r.ctx, (C.mpv_render_update_fn)(cb), userdata)
+	return nil
+}
+
+// Render issues a render call with the provided params (e.g., target FBO, dimensions).
+// The params slice is terminated automatically.
+func (r *RenderContext) Render(params []RenderParam) error {
+	if r == nil || r.ctx == nil {
+		return fmt.Errorf("render context is nil")
+	}
+	cparams := make([]C.mpv_render_param, len(params)+1)
+	for i, p := range params {
+		cparams[i]._type = C.int(p.Type)
+		cparams[i].data = p.Data
+	}
+	cparams[len(params)]._type = C.MPV_RENDER_PARAM_INVALID
+
+	if res := C.mpv_render_context_render(r.ctx, &cparams[0]); res < 0 {
+		return fmt.Errorf("mpv_render_context_render failed: %s", C.GoString(C.mpv_errstr_render(res)))
+	}
+	return nil
+}