Change directory when building FFmpeg on Windows

When building FFmpeg from bash on Windows, we need to make sure we're in the
build directory for the configure, make and make install commands. The pushd
statements are ineffective here as they will be lost once we're in the bash
shell

tablet_qt/tools/build_qt.py

@@ -4267,6 +4267,15 @@ def build_ffmpeg(cfg: Config, target_platform: Platform) -> None:
  ]
  )
 
+ make = MAKE
+
+ make_args = [make]
+ make_install_args = [
+ make,
+ "install",
+ f"DESTDIR={installdir}",
+ ]
+
  if target_platform.windows:
  # We use MSYS/bash because that's what Qt do in their Continuous
  # Integration scripts and we know they work. (choco install msys2)
@@ -4286,30 +4295,36 @@ def build_ffmpeg(cfg: Config, target_platform: Platform) -> None:
  # "cygheap base mismatch detected"
  fail("Ensure msys64\\usr\\bin is before cygwin\\bin in your PATH")
 
- msys_root = Path(shutil.which(MSYS2)).parent.absolute()
- bash = join(msys_root, "usr", "bin", "bash")
- configure_command = " ".join(config_args)
- config_args = [
- bash,
- "-lc",
- f"{configure_command}",
- ]
-
- make = MAKE
-
- make_args = [make]
- make_install_args = [
- make,
- "install",
- f"DESTDIR={installdir}",
- ]
+ config_args = bash_command_args(workdir, config_args)
+ make_args = bash_command_args(workdir, make_args)
+ make_install_args = bash_command_args(workdir, make_install_args)
 
  with pushd(workdir):
  run(config_args, env)
  run(make_args, env)
  run(make_install_args, env)
 
 
+def bash_command_args(workdir: str, command_args: List[str]) -> List[str]:
+ """
+ For the Windows FFmpeg we need to build within bash so all of the
+ configure, make and make install command arguments need to be converted
+ appropriately.
+ """
+ msys_root = Path(shutil.which(MSYS2)).parent.absolute()
+ bash_workdir = workdir.replace("C:", "/c")
+ bash_workdir = bash_workdir.replace("\\", "/")
+ bash = join(msys_root, "usr", "bin", "bash")
+ command = " ".join(command_args)
+ bash_command_args = [
+ bash,
+ "-lc",
+ f"cd {bash_workdir} && {command}",
+ ]
+
+ return bash_command_args
+
+
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 # Eigen
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