[Android] Limit 'max_storage_textures_per_shader_stage' value 4 is better than allowed 0'

[Gordon-F]

Description
Limit 'max_storage_textures_per_shader_stage' value 4 is better than allowed 0'.

Repro steps
Run examples on Android.

Extra materials
max_storage_textures_per_shader_stage on GL:

max_storage_textures_per_shader_stage: vertex_shader_storage_textures
                .min(fragment_shader_storage_textures),

[wgpu-hal/src/gles/adapter.rs:193] vertex_shader_storage_textures = 0
[wgpu-hal/src/gles/adapter.rs:193] fragment_shader_storage_textures = 8

Platform
Android 8.0 (Another device with 11.0), wgpu master.

[Gordon-F]

Same for max_storage_buffers_per_shader_stage:

[wgpu-hal/src/gles/adapter.rs:207] vertex_shader_storage_blocks = 0
[wgpu-hal/src/gles/adapter.rs:208] fragment_shader_storage_blocks = 35

[kvark]

Thank you for filing! We need to change max_storage_textures_per_shader_stage computation to account for some of them could be zeroes (and it's already captured by other downlevel flags).