nv2a: Handle framebuffer CPU blit and PVIDEO only rendering

Handles two edge cases:
1) CPU blits to the framebuffer without using 3D rendering.
2) Fullscreen PVIDEO rendering without any 3D rendering.

In both cases this change prevents the pgraph code from returning early,
bypassing the special case VGA handling in `sdl2_gl_refresh` and instead
using a special framebuffer texture to render the contents of VRAM.

Fixes #652
Fixes #1165

[Tests](https://github.com/abaire/nxdk_pgraph_tests/blob/main/src/tests/antialiasing_tests.cpp)
[HW results](https://github.com/abaire/nxdk_pgraph_tests_golden_results/wiki/Results-Antialiasing_tests)

hw/xbox/nv2a/nv2a_int.h

@@ -263,6 +263,7 @@ typedef struct PGRAPHState {
         GLuint display_size_loc;
         GLuint line_offset_loc;
         GLuint tex_loc;
+        GLuint vga_framebuffer_tex;
         GLuint pvideo_tex;
         GLint pvideo_enable_loc;
         GLint pvideo_tex_loc;