Fix use_texture_alpha for tracks

The use of textures with transparent pixels (for the default tracks: the
"features" on the track) without specifying use_texture_alpha (at least
for the mesh drawtype) has been deprecated in a PR from August 2020[1]
(i.e. since 5.4.0[2]) and removed in the latest dev version[3]. As a
consequence, regular tracks are now rendered with black markers at the
same position where e.g. the blue (Lua)ATC marker would be found.

This commit fixes the issue by enabling use_texture_alpha by default for
all tracks.

[1] https://github.com/minetest/minetest/pull/10122
[2] https://dev.minetest.net/Changelog#5.3.0_.E2.86.92_5.4.0
[3] https://github.com/minetest/minetest/pull/13929

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