[video_core] gate Mesa EXIT terminator detection behind non-proprietary driver (#4151)

FIXES A REGRESSION FROM 4012.

TryFindSize's Mesa fallback matched the unconditional @PT EXIT (0xE30000000007000F) that nv50_ir emits at program end.
That exact word is also emitted by NVN mid-program for unconditional early-outs, so on retail titles (e.g. Super Mario 3D World) the scan truncated at the first early EXIT and dropped the rest of the shader, making textures vanish.

Mesa's terminal EXIT and NVN's mid-program EXIT are bit-identical, so no mask can separate them. Gate the heuristic on !is_proprietary_driver:

NVN binaries (bindless cbuf slot 2) keep self-branch-only sizing, while nouveau/Mesa homebrew (single terminal EXIT, no self-branch trailer) still uses the EXIT terminator. nv50_ir emits exactly one EXIT (early returns lower to BRA-to-exit, discard to DISCARD), so a single terminal match is correct for that path.

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/4151
Reviewed-by: Lizzie <lizzie@eden-emu.dev>
Reviewed-by: MaranBr <maranbr@eden-emu.dev>

src/video_core/shader_environment.cpp

@@ -255,8 +255,7 @@ std::optional<u64> GenericEnvironment::TryFindSize() {
     static constexpr u64 SELF_BRANCH_A = 0xE2400FFFFF87000FULL;
     static constexpr u64 SELF_BRANCH_B = 0xE2400FFFFF07000FULL;
 
-    static constexpr u64 MESA_EXIT_MASK = 0xFFF00000000F001FULL;
-    static constexpr u64 MESA_EXIT_VALUE = (0xE30ULL << 52) | (0x7ULL << 16) | 0xFULL;
+    static constexpr u64 EXIT_VALUE = 0xE30000000007000FULL;
 
     code.resize(MAXIMUM_SIZE / INST_SIZE);
 
@@ -271,7 +270,7 @@ std::optional<u64> GenericEnvironment::TryFindSize() {
             if (inst == SELF_BRANCH_A || inst == SELF_BRANCH_B) {
                 return offset + index;
             }
-            if ((inst & MESA_EXIT_MASK) == MESA_EXIT_VALUE) {
+            if (!is_proprietary_driver && inst == EXIT_VALUE) {
                 return offset + index + INST_SIZE;
             }
         }