Moved interface to separate file

backend_interface.odin

@@ -0,0 +1,25 @@
+package karl2d
+
+Rendering_Backend :: struct {
+	state_size: proc() -> int,
+	init: proc(state: rawptr, window_handle: uintptr, swapchain_width, swapchain_height: int,
+		allocator := context.allocator, loc := #caller_location),
+	shutdown: proc(),
+	clear: proc(color: Color),
+	present: proc(),
+	draw: proc(shader: Shader, texture: Texture_Handle, vertex_buffer: []u8),
+	set_internal_state: proc(state: rawptr),
+
+	load_texture: proc(data: []u8, width: int, height: int) -> Texture_Handle,
+	destroy_texture: proc(handle: Texture_Handle),
+
+	load_shader: proc(shader: string, layout_formats: []Shader_Input_Format = {}) -> Shader,
+	destroy_shader: proc(shader: Shader),
+
+	get_swapchain_width: proc() -> int,
+	get_swapchain_height: proc() -> int,
+
+	set_view_projection_matrix: proc(m: Mat4),
+
+	batch_vertex: proc(v: Vec2, uv: Vec2, color: Color),
+}