Fix Text shader and RenderTexture viewport for SDL2 backend

Text rendering fix:
- Add ShaderType parameter to drawTriangles() to allow explicit shader selection
- Text::draw() now properly uses text shader (alpha-only sampling)
- Previously drawTriangles() always overrode to sprite shader

RenderTexture fix:
- Add pushRenderState/popRenderState to save/restore viewport and projection
- RenderTexture::clear() now sets viewport/projection to FBO dimensions
- RenderTexture::display() restores previous state
- This fixes grid rendering which uses RenderTexture for tile chunks

Co-Authored-By: Claude <noreply@anthropic.com>

src/platform/SDL2Renderer.h

@@ -82,11 +82,16 @@ public:
     // Drawing primitives
     void drawTriangles(const float* vertices, size_t vertexCount,
                        const float* colors, const float* texCoords,
-                       unsigned int textureId = 0);
+                       unsigned int textureId = 0,
+                       ShaderType shaderType = ShaderType::Shape);
 
     // Projection matrix access (for shaders)
     const float* getProjectionMatrix() const { return projectionMatrix_; }
 
+    // Push/pop viewport and projection for RenderTexture
+    void pushRenderState(unsigned int width, unsigned int height);
+    void popRenderState();
+
 private:
     SDL2Renderer() = default;
     ~SDL2Renderer() = default;
@@ -107,6 +112,13 @@ private:
     // FBO stack for nested render-to-texture
     std::vector<unsigned int> fboStack_;
 
+    // Viewport/projection stack for nested rendering
+    struct RenderState {
+        int viewport[4];  // x, y, width, height
+        float projection[16];
+    };
+    std::vector<RenderState> renderStateStack_;
+
     // Helper functions
     bool compileAndLinkProgram(const char* vertexSrc, const char* fragmentSrc, unsigned int& programOut);
     unsigned int compileShaderStage(unsigned int type, const char* source);