[angle] Missing definition for building the GL backend on Windows causes libEGL crash

[amyspark]

Describe the bug

The GL backend requires a few definitions on Windows, which are specified here:

vcpkg/ports/angle/cmake-buildsystem/PlatformWin.cmake

Lines 62 to 64 in 1ba9a25

	list(APPEND ANGLE_DEFINITIONS
	ANGLE_ENABLE_OPENGL
	)

However, there's a critical one missing: https://github.com/google/angle/blob/main/src/libANGLE/renderer/gl/BUILD.gn#L19

This causes the attempt to initialize libEGL to crash with a nonsensical null pointer dereference, because the functions' addresses are never retrieved to begin with: https://bugs.chromium.org/p/angleproject/issues/detail?id=8195

Environment

• OS: [e.g. Windows/Linux etc...] Windows 10 22H2
• Compiler: MSVC 17.6.2

To Reproduce
Steps to reproduce the behavior:

1. ./vcpkg install angle
2. Build Qt 6 with patched ANGLE support from: https://github.com/amyspark/qtbase/tree/amyspark/angle.
3. Build the hellogles3 example against the Qt and ANGLE libraries.

Expected behavior

When the app is launched with the QT_ANGLE_PLATFORM set to opengl, the app should pass libEGL initialization, but be unable to initialize a context. (The latter is a separate ANGLE bug).

Failure logs

See the associated crash on the ANGLE bug tracker.

Additional context

Currently, the app will crash in libEGL initialization.

[FrankXie05]

@amyspark Could you please provide a minimal repro example?
I'll add these options to the GL backend to make sure it works. :)