Added __attribute__ nonnull/pure/const to some GraphX prototypes

[ZERICO2005]

I've added a few function __atrribute__'s to graphx.h. This can help with optimization, and safety.
Changes:

• gfx_AllocSprite and gfx_ConvertToNewRLETSprite take a function pointer to a malloc routine, so I've added __nonnull__ to that parameter since these routines do not check if the function pointer is NULL.
• gfx_Lighten and gfx_Darken are __const__ functions as the same inputs will always generate the same output.
• gfx_GetTextX, gfx_GetTextY, gfx_GetCharWidth, and gfx_GetStringWidth are __pure__ functions, as they read from non-volatile addresses.
• A bunch of the sprite routines say @note sprite_in and sprite_out cannot be the same, so I've added __restrict to both parameters, which can then allow the compiler to warn about it.
• gfx_TilePtr and gfx_TilePtrMapped are __pure__ since they only read from the parameters passed into the function.

Other things that could be done later on:

• gfx_GetPixel might be __pure__, but that depends on if we consider reading from vram to be volatile or not
• __nonnull__ could be applied to most sprite routines, as they don't appear to check if the sprite pointer is NULL
• In the end, graphx.h might look like this with all applicable compiler directives https://github.com/ZERICO2005/toolchain/blob/graphy/src/graphx/graphx.h