respect textures-header-only for embedded textures

[lachbr]

Issue description

The textures-header-only config option is not respected for textures that have been embedded in a bam file. This is likely an oversight, as the common use case for textures-header-only is to reduce memory usage for processes that don't need the texture data, like servers.

Solution description

Simply skip over the bytes containing the image data if textures-header-only is set.

Checklist

I have done my best to ensure that…

• …I have familiarized myself with the CONTRIBUTING.md file
• …this change follows the coding style and design patterns of the codebase
• …I own the intellectual property rights to this code
• …the intent of this change is clearly explained
• …existing uses of the Panda3D API are not broken
• …the changed code is adequately covered by the test suite, where possible.

[codecov]

Codecov Report

Merging #1448 (a196aec) into master (a88b6ee) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1448      +/-   ##
==========================================
- Coverage   16.15%   16.15%   -0.01%     
==========================================
  Files        3745     3745              
  Lines      364122   364098      -24     
==========================================
- Hits        58836    58805      -31     
- Misses     305286   305293       +7     

Impacted Files	Coverage Δ
panda/src/gobj/texture.cxx	13.47% <0.00%> (-0.01%)	⬇️
panda/src/display/nativeWindowHandle.h	56.09% <0.00%> (-7.80%)	⬇️
panda/src/linmath/lvecBase3_src.I	46.78% <0.00%> (-1.85%)	⬇️
panda/src/linmath/lmatrix4_src.I	43.03% <0.00%> (-1.72%)	⬇️
panda/src/express/multifile.I	25.43% <0.00%> (-1.67%)	⬇️
panda/src/linmath/lsimpleMatrix.I	100.00% <0.00%> (ø)
panda/src/device/linuxJoystickDevice.h	100.00% <0.00%> (ø)
panda/src/gobj/geomVertexWriter.I	29.95% <0.00%> (+0.20%)	⬆️
panda/src/pipeline/reMutexDirect.I	54.38% <0.00%> (+5.17%)	⬆️
panda/src/audio/nullAudioManager.h	88.88% <0.00%> (+22.22%)	⬆️
... and 1 more

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[rdb]

Conflicted about this, because it is lossy. Normally you don't lose information because you retain the filename to load from. With this change you can no longer safely roundtrip bam files without losing data.

Can you elaborate further on your use case?

[Maxwell175]

My use case for this PR was saving memory on the headless/server side of things, while still being able to reuse the same bam files as on the client.

[rdb]

That sounds useful in principle, but does textures-header-only actually currently accomplish that? It creates a RAM image filled with a solid blue colour, no?