add load textures functions

for the moment, this file includes only textures loading for opengl.
imgui lacks a working vulkan addImage function.

Signed-off-by: Nicolas Adenis-Lamarre <nicolas.adenis.lamarre@gmail.com>

src/loadTextures.cpp

@@ -0,0 +1,59 @@
+#include <glad/glad.h>
+#define STB_IMAGE_IMPLEMENTATION
+#include "stb_image.h"
+#define STB_IMAGE_RESIZE_IMPLEMENTATION
+#include "stb_image_resize.h"
+
+bool GL_LoadTextureFromFile(const char* filename, GLuint* out_texture, int* out_width, int* out_height, int maxwidth)
+{
+    // load the image
+    int image_width = 0;
+    int image_height = 0;
+    int channels;
+    auto image_data = stbi_load(filename, &image_width, &image_height, &channels, STBI_rgb_alpha);
+    if (!image_data)
+        return false;
+
+    // reduce the image
+    float ratio = 1;
+    if (image_width > maxwidth && maxwidth != 0) {
+        ratio = maxwidth / static_cast<float>(image_width);
+    }
+
+    int image_width_resized  = image_width * ratio;
+    int image_height_resized = image_height * ratio;
+    unsigned char* image_data_resized = (unsigned char*)malloc(image_width_resized * image_height_resized * 4);
+    if (!image_data_resized) {
+        stbi_image_free(image_data);
+        return false;
+    }
+
+    stbir_resize_uint8(image_data, image_width, image_height, 0,
+                        image_data_resized, image_width_resized, image_height_resized, 0,
+                        4);
+    stbi_image_free(image_data);
+
+    // Create a OpenGL texture identifier
+    GLuint image_texture;
+    glGenTextures(1, &image_texture);
+    glBindTexture(GL_TEXTURE_2D, image_texture);
+
+    // Setup filtering parameters for display
+    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
+    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
+    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE); // This is required on WebGL for non power-of-two textures
+    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE); // Same
+
+    // Upload pixels into texture
+    #if defined(GL_UNPACK_ROW_LENGTH) && !defined(__EMSCRIPTEN__)
+    glPixelStorei(GL_UNPACK_ROW_LENGTH, 0);
+    #endif
+    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, image_width_resized, image_height_resized, 0, GL_RGBA, GL_UNSIGNED_BYTE, image_data_resized);
+    stbi_image_free(image_data_resized);
+
+    *out_texture = image_texture;
+    *out_width   = image_width_resized;
+    *out_height  = image_height_resized;
+
+    return true;
+}