vo_opengl: angle: dynamically load ANGLE

ANGLE is _really_ annoying to build. (Requires special toolchain and a
recent MSVC version.) This results in various issues with people
having trouble to build mpv against ANGLE (apparently linking it
against a prebuilt binary doesn't count, or using binaries from
potentially untrusted sources is not wanted).

Dynamically loading ANGLE is going to be a huge convenience. This commit
implements this, with special focus on keeping it source compatible to
a normal build with ANGLE linked at build-time.

1 files changed, 33 insertions, 0 deletions

diff --git a/video/out/opengl/angle_dynamic.c b/video/out/opengl/angle_dynamic.c
new file mode 100644
index 0000000000..f4540c473a
--- /dev/null
+++ b/video/out/opengl/angle_dynamic.c
@@ -0,0 +1,33 @@
+#include <pthread.h>
+#include <windows.h>
+
+#define ANGLE_NO_ALIASES
+#include "angle_dynamic.h"
+
+#include "common/common.h"
+
+#define ANGLE_DECL(NAME, VAR) \
+    VAR;
+ANGLE_FNS(ANGLE_DECL)
+
+static bool angle_loaded;
+static pthread_once_t angle_load_once = PTHREAD_ONCE_INIT;
+
+static void angle_do_load(void)
+{
+    // Note: we let this handle "leak", as the functions remain valid forever.
+    HANDLE angle_dll = LoadLibraryW(L"LIBEGL.DLL");
+    if (!angle_dll)
+        return;
+#define ANGLE_LOAD_ENTRY(NAME, VAR) \
+    MP_CONCAT(PFN_, NAME) = (void *)GetProcAddress(angle_dll, #NAME); \
+    if (!MP_CONCAT(PFN_, NAME)) return;
+    ANGLE_FNS(ANGLE_LOAD_ENTRY)
+    angle_loaded = true;
+}
+
+bool angle_load(void)
+{
+    pthread_once(&angle_load_once, angle_do_load);
+    return angle_loaded;
+}