debug performance issue

[nopenotj]

use perf to profile code.

need to compile with -fno-omit-frame-pointer -ggdb
refer to https://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html

[nopenotj]

"For example, you'll reach the limits of your graphics card very quickly if you draw a large number of sprites. The reason is that performance depends in large part on the number of calls to the draw function. Indeed, each call involves setting a set of OpenGL states, resetting matrices, changing textures, etc. All of this is required even when simply drawing two triangles (a sprite). This is far from optimal for your graphics card: Today's GPUs are designed to process large batches of triangles, typically several thousand to millions.

To fill this gap, SFML provides a lower-level mechanism to draw things: Vertex arrays. As a matter of fact, vertex arrays are used internally by all other SFML classes. They allow for a more flexible definition of 2D entities, containing as many triangles as you need. They even allow drawing points or lines." (https://www.sfml-dev.org/tutorials/2.5/graphics-vertex-array.php)

Need to convert tilemap to vertex array