[rtextures] Adding triangle drawing function for images

[Bigfoot71]

New functions added

void ImageDrawTriangle(Image *dst, Vector2 v1, Vector2 v2, Vector2 v3, Color color);
void ImageDrawTriangleEx(Image *dst, Vector2 v1, Vector2 v2, Vector2 v3, Color c1, Color c2, Color c3);
void ImageDrawTriangleLines(Image *dst, Vector2 v1, Vector2 v2, Vector2 v3, Color color);
void ImageDrawTriangleFan(Image *dst, Vector2 *points, int pointCount, Color color);
void ImageDrawTriangleStrip(Image *dst, Vector2 *points, int pointCount, Color color);

Why and how

1. These functions enable the rendering of triangles on images, supporting any vertex order (CW/CCW).

2. The ImageDrawTriangleEx function allows the rendering of a triangle with vertex colors specified for interpolation. While not strictly necessary, it serves as a reference for those seeking a more comprehensive function and understanding of its operation.

3. Additional functions ImageDrawTriangleLines, ImageDrawTriangleFan, and ImageDrawTriangleStrip were introduced to align with rshapes.c.

4. The chosen rendering method is the barycentric test method. This method was preferred over the scanline method due to its simplicity and the ease with which pixel processing can be parallelized, potentially resulting in significant efficiency gains when implemented correctly.

Basic example

#include <raylib.h>

typedef struct {
    Vector2 p1, p2, p3;
    Color c1, c2, c3;
} Triangle;

int main(void)
{
    InitWindow(800, 600, "Raysterized");

    // CW order
    Triangle t1 = {
        (Vector2) { 0, 0 },
        (Vector2) { 800, 0 },
        (Vector2) { 0, 600 },
        (Color) { 255, 0, 0, 255 },
        (Color) { 0, 255, 0, 255 },
        (Color) { 0, 0, 255, 255 },
    };

    // CCW order
    Triangle t2 = {
        (Vector2) { 800, 0 },
        (Vector2) { 0, 600 },
        (Vector2) { 800, 600 },
        RED
    };

    Image image = GenImageColor(800, 600, BLACK);

    ImageDrawTriangleEx(&image, t1.p1, t1.p2, t1.p3, t1.c1, t1.c2, t1.c3);
    ImageDrawTriangle(&image, t2.p1, t2.p2, t2.p3, t2.c1);

    Texture texture = LoadTextureFromImage(image);

    while (!WindowShouldClose())
    {
        BeginDrawing();
        ClearBackground(RAYWHITE);
        DrawTexture(texture, 0, 0, WHITE);
        EndDrawing();
    }

    CloseWindow();

    return 0;
}

[raysan5]

@Bigfoot71 Hey! This is a nice addition! Is it ready to merge?

[Bigfoot71]

@raysan5 I just fixed two small mistakes in ImageDrawTriangleFan and ImageDrawTriangleStrip.
Everything is tested and working, you can merge if you want! :)

[raysan5]

@Bigfoot71 Nice! Thanks for this addition! 👍😄