Change when/how movie length is calculated

@alyosha-tas brought this up in #2197. There are actually a few reasons for this. One main reason is that loading movies for previewing currently requires loading the entire movie file so the frame count can be displayed. There are two potential solutions here: add a frame count field to the header so the movie's frame count can be previewed without excessive loading. Or just don't display the frame count in the preview.

Besides the basic slowness of loading entire movies just for the purpose of previewing them, I think there's some double-loading going on for some reason. Each movie appears to be loaded twice from some preliminary debugging.

Note: The double-loading is caused by the initial hash check loading the movies to check if the hash matches the current game (PlayMovie.MatchHashCheckBox_CheckedChanged), and then the movies actually being loaded for display (PlayMovie.PlayMovie_Load). (fixed in fe845ce8efae8fa3b11d04079d3ae20ec844ea8a)

2024-10-03 note: this is largely low priority for me now because of some efficiency improvements in other parts of the emulator since this issue was originally filed.

Start by reading the PlayMovie.MatchHashCheckBox_CheckedChanged and PlayMovie.PlayMovie_Load entry points, noting that the double-loading was fixed in fe845ce8efae8fa3b11d04079d3ae20ec844ea8a. Determine whether the intended outcome is header-based frame counts or hiding frame counts in previews, then verify that movie previews no longer require excessive loading.