[DRAFT] Add external texture support (Vulkan)

[dsnopek]

This builds on top of PR #96982 to add external texture support for Vulkan.

This is an unfinished work-in-progress.

I ended up splitting this off into its own PR to finish later after I discovered that this will only work with Android API level 26 or higher. Presently, we have an API minimum of 21, which prevents this code from compiling.

So, I'll save this until after we increase the API minimum to 21 or higher. (Another option is including the code, but then requiring users to build with scons ndk_platform=android-26 in order to support this feature, but it wouldn't be included in official builds.)

[m4gr3d]

According to https://composables.com/android-distribution-chart, we go from 99.7% to 95.4% coverage if we raise the min API from 21 to 26.

[akien-mga]

(Another option is including the code, but then requiring users to build with scons ndk_platform=android-26 in order to support this feature, but it wouldn't be included in official builds.)

I think we can do that, it might take a while for us to bump min version to android-26 I fear, given the current metrics. Phasing out old devices is taking a very long time, and a number of users would be upset if we cut off 5% of the addressable market.

We could alternatively consider bumping the current ndk_platform, but still keep support for android-21 when compiling custom export templates for users who want it.

A third option, but which requires more discussion and work, could be to provide separate export templates for OpenGL/Compatibility (with Vulkan/RD code removed, and supporting android-21) and for Vulkan (with min API on android-26). That would help reduce the binary size for users who want to target the lower end of the market, and probably help cull a lot of devices with completely broken Vulkan drivers for the Vulkan build.

[dsnopek]

I think we can do that, it might take a while for us to bump min version to android-26 I fear, given the current metrics. Phasing out old devices is taking a very long time, and a number of users would be upset if we cut off 5% of the addressable market.

That makes sense!

We discussed this at the XR team meeting last night (as the primary use case for this is ARCore support), and one solution we discussed was having the 'godot_arcore' project distribute its own Android export templates that were compiled with scons ndk_platform=android-26, since folks will already need to download the ARCore GDExtension from there.

I personally like this solution: it keeps Godot's base Android support widely compatible (which is what is needed by most people), while having all the tools for doing ARCore (which is a much less common use-case) easily available for those who need them.