vo_gpu: greatly increase maximum shader cache size

See #8137 for justification.

This is not ideal, because a large cache results in a lot of `strcmp`
invocations for every single shader invocation. But it's way better than
resulting in a lot of shader recompilations for every single shader
invocation.

The only reason I don't want to uncap it altogether is because there are
conceivable edge cases in which users load dynamically generated shaders
with updated parameters (indeed, I've seen IRC discussions to this
effect), and in this case, we don't want to grow the cache infinitely as
a result of something like a floating point parameter being continuously
updated. (Never mind that this *would* actually trigger worst case
behavior for the `strcmp`, since the updated float constant is likely to
be near the bottom of the shader body)

Whatever. vo_placebo will liberate us all in the end.

video/out/gpu/shader_cache.c

@@ -17,7 +17,7 @@
 #include "utils.h"
 
 // Force cache flush if more than this number of shaders is created.
-#define SC_MAX_ENTRIES 48
+#define SC_MAX_ENTRIES 256
 
 union uniform_val {
     float f[9];         // RA_VARTYPE_FLOAT