feat: Add initial Vulkan backend framework and compile flag

I've introduced the foundational infrastructure for a Vulkan rendering backend in raylib.

Key changes:
- CMake:
    - I added a `SUPPORT_VULKAN` option (default OFF) to enable Vulkan.
    - It finds the Vulkan SDK and links appropriate libraries when enabled.
    - I defined `CF_VULKAN_` (0 by default, 1 if Vulkan enabled) and `GRAPHICS_API_VULKAN` preprocessor macros.
    - I updated `LibraryConfigurations.cmake` and `src/CMakeLists.txt` to handle Vulkan as a graphics API.
- Vulkan Abstraction Layer:
    - I created `src/rlvk.h` and `src/rlvk.c` with stub implementations for Vulkan rendering functions (e.g., `rlvkInit`, `rlvkClose`, drawing functions). These are compiled only when `SUPPORT_VULKAN` is ON.
- Core Integration:
    - `rlgl.h` and `rcore.c` now have conditional code paths for `GRAPHICS_API_VULKAN`.
    - `InitWindow` and `CloseWindow` in `rcore.c` call Vulkan-specific platform and backend initialization/deinitialization stubs.
- Platform Layer (GLFW):
    - `src/platforms/rcore_desktop_glfw.c` includes stubbed `InitPlatformVulkan` and `ClosePlatformVulkan` to set up GLFW for a Vulkan context (actual Vulkan instance/surface creation is stubbed).

This commit establishes the necessary build system changes and C code structure to allow for the incremental implementation of the Vulkan renderer. Currently, enabling Vulkan will compile the stubs, allowing your application to run but without actual Vulkan rendering. The `CF_VULKAN_` flag controls the compilation and selection of Vulkan as the default renderer when active.

src/rlvk.c

@@ -0,0 +1,77 @@
+#include "rlvk.h"
+#include "utils.h"      // For TRACELOG if needed
+#include <stdio.h>      // For printf in stubs
+#include <stdlib.h>     // For RL_MALLOC, RL_FREE if used
+
+// Global or static variables for Vulkan state (minimal for stubs)
+static bool rlvkReady = false;
+
+void rlvkInit(VkInstance instance, VkSurfaceKHR surface, int width, int height) {
+    printf("rlvkInit called (STUB)\n");
+    // Minimal check or setup
+    if (instance != VK_NULL_HANDLE && surface != VK_NULL_HANDLE) {
+        rlvkReady = true;
+        TRACELOG(LOG_INFO, "RLVK: Vulkan backend initialized (stubbed).");
+    } else {
+        TRACELOG(LOG_ERROR, "RLVK: Failed to initialize Vulkan backend due to null instance or surface (stubbed).");
+        rlvkReady = false;
+    }
+}
+
+void rlvkClose(void) {
+    printf("rlvkClose called (STUB)\n");
+    rlvkReady = false;
+    TRACELOG(LOG_INFO, "RLVK: Vulkan backend closed (stubbed).");
+}
+
+bool rlvkIsReady(void) {
+    return rlvkReady;
+}
+
+void rlvkBeginDrawing(void) {
+    // printf("rlvkBeginDrawing called (STUB)\n");
+}
+
+void rlvkEndDrawing(void) {
+    // printf("rlvkEndDrawing called (STUB)\n");
+}
+
+void rlvkClearBackground(unsigned char r, unsigned char g, unsigned char b, unsigned char a) {
+    // printf("rlvkClearBackground called (STUB) with color: %u, %u, %u, %u\n", r, g, b, a);
+}
+
+unsigned int rlvkLoadTexture(const void *data, int width, int height, int format, int mipmaps) {
+    printf("rlvkLoadTexture called (STUB)\n");
+    return 0; // Return a dummy ID
+}
+
+void rlvkUnloadTexture(unsigned int id) {
+    printf("rlvkUnloadTexture called (STUB) for ID: %u\n", id);
+}
+
+unsigned int rlvkLoadShaderCode(const char *vsCode, const char *fsCode) {
+    printf("rlvkLoadShaderCode called (STUB)\n");
+    return 0; // Return a dummy ID
+}
+
+void rlvkUnloadShaderProgram(unsigned int id) {
+    printf("rlvkUnloadShaderProgram called (STUB) for ID: %u\n", id);
+}
+
+int rlvkGetLocationUniform(unsigned int shaderId, const char *uniformName) {
+    // printf("rlvkGetLocationUniform called (STUB) for shader ID: %u, uniform: %s\n", shaderId, uniformName);
+    return -1;
+}
+
+int rlvkGetLocationAttrib(unsigned int shaderId, const char *attribName) {
+    // printf("rlvkGetLocationAttrib called (STUB) for shader ID: %u, attrib: %s\n", shaderId, attribName);
+    return -1;
+}
+
+void rlvkSetUniform(int locIndex, const void *value, int uniformType, int count) {
+    // printf("rlvkSetUniform called (STUB) for locIndex: %d\n", locIndex);
+}
+
+// ... other rlgl equivalent function stub implementations ...
+// TRACELOG can be used for more detailed stub logging if utils.h is appropriately included and linked.
+// For now, simple printf might be fine for basic stub verification.