docs: update README with SDL3 backend details and smoke test instructions

Expand the README to include information on the new SDL3 + SDL_GPU backend for `imgui-app`, configuration example, and instructions for running the smoke test using `MainSdl`. Adjust `example/build.gradle` to support dynamic main class selection via `-PmainClass`.

Author: SpaiR

README.md

@@ -47,6 +47,8 @@ consuming them as a dependency only needs JDK 8+ at runtime.
 git clone --recurse-submodules https://github.com/SpaiR/imgui-java.git
 cd imgui-java
 ./gradlew :example:run
+# SDL3 + SDL_GPU backend smoke test:
+./gradlew :example:run -PmainClass=MainSdl
 ```
 
 `example/src/main/java/` contains a runnable showcase for every widget and extension — start with `Main.java`, then
@@ -64,6 +66,30 @@ Pick a module based on how much control you need:
 On Apple Silicon nothing extra is needed: the published macOS native is a universal binary covering both `x86_64` and
 `aarch64`.
 
+### SDL3 backend (imgui-app)
+
+`imgui-app` now supports two backends via `Configuration#setBackend(...)`:
+
+- `Backend.GLFW` (default)
+- `Backend.SDL` (SDL3 + SDL_GPU)
+
+Use SDL3 by switching backend in `configure(...)`:
+
+```
+@Override
+protected void configure(final Configuration config) {
+    config.setBackend(Backend.SDL);
+}
+```
+
+For a ready smoke-test entry point, run `MainSdl`:
+
+```
+./gradlew :example:run -PmainClass=MainSdl
+```
+
+At the moment, SDL backend in `imgui-app` is focused on single-window flow (multi-viewport is not supported).
+
 ## Application
 
 If you don't care about OpenGL and other low-level stuff, then you can use application layer from `imgui-app` module.

example/build.gradle

@@ -9,7 +9,7 @@ java {
 }
 
 application {
-    mainClass = 'Main'
+    mainClass = (findProperty('mainClass') ?: 'Main') as String
     try {
         if (libPath) {
             applicationDefaultJvmArgs = ["-Dimgui.library.path=$libPath"]