glTF exporter: Manage alpha when performing RGB2BW conversion

Based on what is visually done in viewport

scripts/addons_core/io_scene_gltf2/__init__.py

@@ -5,7 +5,7 @@
 bl_info = {
     'name': 'glTF 2.0 format',
     'author': 'Julien Duroure, Scurest, Norbert Nopper, Urs Hanselmann, Moritz Becher, Benjamin Schmithüsen, Jim Eckerlein, and many external contributors',
-    "version": (4, 2, 71),
+    "version": (4, 2, 72),
     'blender': (4, 2, 0),
     'location': 'File > Import-Export',
     'description': 'Import-Export as glTF 2.0',

scripts/addons_core/io_scene_gltf2/blender/exp/material/extensions/gltf2_blender_image.py

@@ -350,6 +350,8 @@ class ExportImage:
                     out_buf[int(dst_chan)::4] = fill.value
                 elif isinstance(fill, FillImageRGB2BW) and fill.image == image:
                     out_buf[int(dst_chan)::4] = tmp_buf[0::4] * 0.2989 + tmp_buf[1::4] * 0.5870 + tmp_buf[2::4] * 0.1140
+                    if image.alpha_mode in ["STRAIGHT", "PREMUL"]:
+                        out_buf[int(dst_chan)::4] *= tmp_buf[3::4]
 
         tmp_buf = None  # GC this