ebiten: remove vertex copying in DrawTriangles() / DrawTrianglesShader() loop

[SolarLune]

Ebitengine Version

v2.8.0-alpha.9 (?) / latest master

Operating System

• Windows
• macOS
• Linux
• FreeBSD
• OpenBSD
• Android
• iOS
• Nintendo Switch
• PlayStation 5
• Xbox
• Web Browsers

Go Version (go version)

1.23.0

What steps will reproduce the problem?

This is just an issue tracking a small optimization for Image.DrawTriangles() and Image.DrawTrianglesShader(). Internally, these functions loop through a slice of vertices and copy their contents to an internal vertex data slice before proceeding with rendering. This can be seen here:

https://github.com/hajimehoshi/ebiten/blob/main/image.go#L721

Currently, this is done with for i, v := range vertices - by using this version of a for loop, the vertex data is copied into v. This is an unnecessary step, since we're then just copying the data directly into the internal vertex slice.

This should probably be replaced with for i := range vertices, after which we can just index the vertices slice to get the data.

In my limited testing and profiling, adjusting the loop this makes the loop take about 13% less time. Timing the loop itself went from ~216µs to ~188µs per DrawTrianglesShader() call with a relatively large number of vertices (about 6000 triangles per call).

What is the expected result?

N/A

What happens instead?

N/A

Anything else you feel useful to add?

N/A