[3.0] Fix bug in Image::_get_pixelb

Fixes issue:
 https://github.com/godotengine/godot/issues/8158

core/image.cpp

@@ -94,7 +94,7 @@ void Image::_get_pixelb(int p_x, int p_y, uint32_t p_pixelsize, const uint8_t *p
 	uint32_t ofs = (p_y * width + p_x) * p_pixelsize;
 
 	for (uint32_t i = 0; i < p_pixelsize; i++) {
-		p_dst[ofs] = p_src[ofs + i];
+		p_dst[i] = p_src[ofs + i];
 	}
 }