Key repeat for nk_keys

[PROP65]

Adds key repeat for nk_keys (Arrows, Home, End, Backspace, Delete, etc).

The default delay and interval can be changed, as well as updated at runtime.

/* send 20 inputs per second after a key has been held for 1/3 of a second */
#define NK_INPUT_REPEATER_DELAY 0.33f
#define NK_INPUT_REPEATER_INTERVAL 0.05f
#define NK_KEY_REPEAT
#include "nuklear.h"

...

struct nk_context ctx;
nk_init_default(&ctx, 0);
/* update at runtime, 10 inputs per second after 0.4 seconds */
ctx.input.keyboard.repeater_delay    = 0.40f;
ctx.input.keyboard.repeater_interval = 0.10f;

Tested and working with the following render backends.

• glfw_opengl2
• glfw_opengl3
• glfw_vulkan

[RobLoach]

Nice work! Looking good... Adding delta time to the renders will help too #628

[PROP65]

I've reworked this patch to make the key repeat functionality more generic/reusable.
Most of the changes have been moved to nuklear_input.c and a new function is provided (nk_input_is_key_fired) to check the new boolean.

After submitting this pull request I realized some backends have key repeat via their abstraction layer (SDL, X11, etc) so this functionality is also now opt-in and can be enabled by defining NK_KEY_REPEAT before including Nuklear

...
#define NK_KEY_REPEAT
#include "nuklear.h"
...