[r] fix trackplot bugs with small tracks/motifs

[immanuelazn]

Description

Various small changes to coverage plots and genome peak plots to handle smaller motifs better.

• Created at least a single arrow for peaks that are not long enough for the pre-determined minimum arrow segment length. Note, in very small peaks the arrow may go out of bounds of the peak itself, which will likely also need to be fixed in the future.
• Fixed an issue where the head_only boolean flow in trackplot_create_arrow_segs() did not properly assign values to the start and end position if a strand was negative.
• Fixed an issue arising from tracks smaller than 500 bp in trackplot_coverage()

Tests

• Added a small test case for the head_only boolean flow in trackplot_create_arrow_segs()
• Added tests for small regions in trackplot_coverage()

[immanuelazn]

For peaks with no arrows: Before

After

Code to run:

region <- GenomicRanges::GRanges(
  seqnames = S4Vectors::Rle(c("chr15")),
  ranges = IRanges::IRanges(52789700:52790000)
)
trackplot_genome_annotation(
  loci=demo_small_motifs$hits$Eye_c11,
  region=region,
  annotation_size=3,
  show_strand=TRUE,
  color_by="score",
  label_by="name",
  num_arrows=50
)