Disable VAO cache under Emscripten

[jdarpinian]

The VAO surface cache uses bufferSubData and triggers a very slow path in some GLES implementations. Specifically I have observed 10x frame times with ANGLE/Metal on macOS and with Mali on Android.

Of course disabling the cache is not ideal. But at least this fixes wildly inconsistent frame times for now.

[zturtleman]

The VAO cache is slow on practically everything except Nvidia's driver. (The handling could use being reworked.)

• You can remove the cvar from code/renderergl1/
• Rename r_disableStaticSurfaceVaoCache to r_vaoCache (and invert the usage, if (!r_vaoCache->integer))
• Default it to 0 for all platforms (disabling the VAO cache)
• Remove the code comment explaining changing it for Emscripten

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OpenGL2: Disable the VAO cache by default

The VAO surface cache uses glBufferSubData and triggers a very slow path
in some GLES implementations. Specifically I have observed 10x frame
times under Emscripten with ANGLE/Metal on macOS and with Mali on
Android.

[zturtleman]

Fixed via #723.