Error building: gtk.h: No such file or directory

[bbhtt]

While trying to update the Flatpak, Hexgl from the 0.2.0 tag fails to build with this error:

The Meson build system
Version: 0.59.4
Source dir: /run/build/gnome-hexgl
Build dir: /run/build/gnome-hexgl/_flatpak_build
Build type: native build
Project name: gnome-hexgl
Project version: 0.2.0
C compiler for the host machine: ccache cc (gcc 11.3.0 "cc (GCC) 11.3.0")
C linker for the host machine: cc ld.bfd 2.37
Host machine cpu family: x86_64
Host machine cpu: x86_64
Found pkg-config: /usr/bin/pkg-config (1.8.0)
Run-time dependency gthree-1.0 found: YES 1.0
Run-time dependency gsound found: YES 1.0.3
Library m found: YES
WARNING: Project targeting '>= 0.48.0' but tried to use feature introduced in '0.49.0': configure_file.configuration dictionary.
Configuring wrapper.sh using configuration
Build targets in project: 1
WARNING: Project specifies a minimum meson_version '>= 0.48.0' but uses features which were added in newer versions:
 * 0.49.0: {'configure_file.configuration dictionary'}

Found ninja-1.10.2 at /usr/bin/ninja
[1/12] Compiling C object gnome-hexgl.p/src_hexgl.c.o
FAILED: gnome-hexgl.p/src_hexgl.c.o 
ccache cc -Ignome-hexgl.p -I. -I.. -I/app/include/gthree-1.0 -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I/usr/include/sysprof-4 -I/app/include/graphene-1.0 -I/app/lib/graphene-1.0/include -I/usr/include/json-glib-1.0 -I/usr/include/libmount -I/usr/include/blkid -fdiagnostics-color=always -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -std=gnu99 -O2 -g '-DDATADIR="/app/share/gnome-hexgl"' -O2 -g -pipe -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -fno-omit-frame-pointer -pthread -D_REENTRANT -mfpmath=sse -msse -msse2 -MD -MQ gnome-hexgl.p/src_hexgl.c.o -MF gnome-hexgl.p/src_hexgl.c.o.d -o gnome-hexgl.p/src_hexgl.c.o -c ../src/hexgl.c
../src/hexgl.c:2:10: fatal error: gtk/gtk.h: No such file or directory
    2 | #include <gtk/gtk.h>
      |          ^~~~~~~~~~~
compilation terminated.
[2/12] Compiling C object gnome-hexgl.p/src_utils.c.o
FAILED: gnome-hexgl.p/src_utils.c.o 
ccache cc -Ignome-hexgl.p -I. -I.. -I/app/include/gthree-1.0 -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I/usr/include/sysprof-4 -I/app/include/graphene-1.0 -I/app/lib/graphene-1.0/include -I/usr/include/json-glib-1.0 -I/usr/include/libmount -I/usr/include/blkid -fdiagnostics-color=always -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -std=gnu99 -O2 -g '-DDATADIR="/app/share/gnome-hexgl"' -O2 -g -pipe -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -fno-omit-frame-pointer -pthread -D_REENTRANT -mfpmath=sse -msse -msse2 -MD -MQ gnome-hexgl.p/src_utils.c.o -MF gnome-hexgl.p/src_utils.c.o.d -o gnome-hexgl.p/src_utils.c.o -c ../src/utils.c
In file included from /app/include/gthree-1.0/gthree/gthreeuniforms.h:8,
                 from /app/include/gthree-1.0/gthree/gthreeshader.h:7,
                 from /app/include/gthree-1.0/gthree/gthreeprogram.h:8,
                 from /app/include/gthree-1.0/gthree/gthreematerial.h:10,
                 from /app/include/gthree-1.0/gthree/gthreemeshmaterial.h:8,
                 from /app/include/gthree-1.0/gthree/gthree.h:11,
                 from ../src/utils.h:1,
                 from ../src/utils.c:1:
/app/include/gthree-1.0/gthree/gthreetexture.h:9:10: fatal error: gdk-pixbuf/gdk-pixbuf.h: No such file or directory
    9 | #include <gdk-pixbuf/gdk-pixbuf.h>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
ninja: build stopped: subcommand failed.
Error: module gnome-hexgl: Child process exited with code 1

However the headers are present in the runtime/SDK.

Building from current master f47a351 works fine

Any idea? With gthree 0.9.0 we'd need master of hexgl anyway, since this isn't included edf6a75 but this particular build failure seems unrelated to gthree.

The manifest used (which fails) org.gnome.HexGL.txt