implemented new linear gradient generation function (#4462)

Author: decromo

src/rtextures.c

@@ -827,16 +827,27 @@ Image GenImageGradientLinear(int width, int height, int direction, Color start,
     float cosDir = cosf(radianDirection);
     float sinDir = sinf(radianDirection);
 
+    // Calculate how far the top-left pixel is along the gradient direction from the center of said gradient
+    float startingPos = 0.5 - (cosDir*width/2) - (sinDir*height/2);
+    // With directions that lie in the first or third quadrant (i.e. from top-left to 
+    // bottom-right or vice-versa), pixel (0, 0) is the farthest point on the gradient
+    // (i.e. the pixel which should become one of the gradient's ends color); while for
+    // directions that lie in the second or fourth quadrant, that point is pixel (width, 0).
+    float maxPosValue = 
+            ((signbit(sinDir) != 0) == (signbit(cosDir) != 0))
+            ? fabs(startingPos)
+            : fabs(startingPos+width*cosDir);
     for (int i = 0; i < width; i++)
     {
         for (int j = 0; j < height; j++)
         {
             // Calculate the relative position of the pixel along the gradient direction
-            float pos = (i*cosDir + j*sinDir)/(width*cosDir + height*sinDir);
+            float pos = (startingPos + (i*cosDir + j*sinDir)) / maxPosValue;
 
             float factor = pos;
-            factor = (factor > 1.0f)? 1.0f : factor;  // Clamp to [0,1]
-            factor = (factor < 0.0f)? 0.0f : factor;  // Clamp to [0,1]
+            factor = (factor > 1.0f)? 1.0f : factor;  // Clamp to [-1,1]
+            factor = (factor < -1.0f)? -1.0f : factor;  // Clamp to [-1,1]
+            factor = factor / 2 + 0.5f;
 
             // Generate the color for this pixel
             pixels[j*width + i].r = (int)((float)end.r*factor + (float)start.r*(1.0f - factor));